For Everyone with Bigger Tires
JasIzBac
03-10-2002, 01:40 AM
I DIDNT WRITE THIS, BUT I GOT THIS OFF SOME SITE. I DIDNT READ THE WHOLE THING THOUGH :)
GEAR SWAPS
Bigger tires are on top of every must-have list. But larger tires make a gear change necessary. Gearing improvements can be done to the axles, transmission or transfer case. There are advantages to each depending on the results you're trying to achieve. Some people may be looking for the ultimate crawl ratio; others may just want to get the overall gearing back to where it was before the todious meats were installed. Then there are those who want to match a specific engine rpm to a certain road speed. Since everyone's needs and mechanical combinations are different, there's no magical chart yielding the perfect gearing. This story will tell you how to make the right choices for your 4x4. It will help if you read the "Gearhead Math" and "Crawl Ratio" sidebars first.
AXLE GEARS
We get a ton of questions about axle gearing versus tire size, so let's start with extreme basics: Adding taller tires or higher-ratio gears reduces engine rpm at all road speeds and gives the truck less mechanical advantage to help it crawl over an obstacle or get moving from a stop. Smaller tires or lower gears increase engine rpm and acceleration/ crawling capabilities. Low gears are higher numerically (4.10 is lower than 3.50) and high gears are lower numerically (3.50 is higher than 4.10).
Once you understand those basics, you'll see why adding taller tires can cause the engine to lug below its powerband and cause poor performance. To fix the problem, you need to swap to lower-ratio axle gears. The "Tall-Tire Short Cut" sidebar tells you how to regear for stock performance. But many 'wheelers want gears that make the overall combination even lower than stock to improve towing or off-highway prowess. Others may have highway-bound 4x4s that turn too much rpm on the freeway and need higher-ratio axle gears to lower the engine rpm. You can make small changes to the truck's performance with tire sizing: Shorter tires act like lower gears (more highway rpm) and taller tires act like higher gears (less highway rpm).
The good and bad news about axle-gear swaps and tire-size changes is that they affect performance in any trans gear and in either high range or low range. That's good because a simple swap is all it takes to restore performance to what it was before you put on tall tires. The bad news is that very low gears for dirt performance may cause more highway rpm than you can live with, and swapping to high gears for cruising at speed will hamper towing and off-highway ability. When balancing both ends of the scale, your gearing choices become fairly limited. That's why proper axle-gearing choices must be made with the transmission and T-case gears in mind. In many cases the cost of new gears might be as much as an overdrive tranny swap that would give you the best of both worlds. Read on...
TRANSMISSION RATIOS
Tranny swaps are for 'wheelers who want more tuned gear-ratio performance than is possible with axle-gear swaps. A trans swap can net either a very-low First gear or an overdrive--or in the case of the NV4500 trans, both. If you don't have an overdrive tranny and are happy with the highway rpm of your current axle gears, then a trans with a lower First-gear ratio can provide the low-speed trail performance (crawl ratio) you're looking for. Alternately, if you like your current low-geared axles for 'wheeling but hate the rpm on the freeway, then you'll want to swap to an overdrive tranny--usually an automatic with a .70:1 Fourth gear or a manual with a .75:1 Fifth--to drop engine rpm at high road speeds. High axle gears and a low First gear or low axle gears with an overdrive are different means to the same end.
Ready to spend more bucks for performance? Consider this: If you're happy with your freeway cruise rpm but want a lower low range, swap in an overdrive tranny and lower axle gears. If you calculate the ratios to achieve the same freeway rpm you had without overdrive, then the lower axle gears will give you a lower crawl ratio at no cost to highway speed. Many overdrive trannies also have a lower First gear, so you win on both ends. Do the math and you can come up with some really interesting combinations.
However, tranny swaps can have drawbacks. They won't cure the problem of tall tires and too-high axle gears because they usually have only a lower First gear, so while you may fix the stoplight acceleration problem, you still won't be able to hold speed on the highway. Also, just as with axle-ratio swaps, tranny changes affect performance whether you're in high range or low range. If you swapped in a granny-low four-speed you'll have to live with it on the street. If the trans isn't geared low enough in Second, or if your engine doesn't make enough power to pull away from a stoplight in Second, then you'll get really irritated having to row through the ultralow First. In some cases First may not have synchromesh.
You also have to consider how far rpm will fall between gears. Calculate your road speed and rpm in each gear or else you may end up with a Fourth gear that lugs the engine at your cruise rpm but a Third that runs at too much rpm. This is especially a problem with small engines, heavy vehicles and towing. Automatics are less of a worry than manuals on this score, but keep in mind that many four-speed autos should not be operated in overdrive for towing. And if you swap an aftermarket low First/Second gearset into an automatic, be careful that the rpm drop from Second to Third isn't too great.
TRANSFER CASE
The latest trend in gearing is to alter the low-range ratio of the transfer case by either changing internal ratios or mating boxes to double the low-range ratio (see "Learning to Crawl" elsewhere in this issue). This is a particularly interesting solution for 'wheelers who want the lowest gearing possible yet still drive on the street. Changing the low-range ratio won't alter the high-range performance (except in Samurais because high range is also lowered) so there are no on-highway compromises, but you have to sort out your axle ratios before you begin. A low T-case can omit your need for an overdrive tranny if you gear the axles for a perfect cruise rpm and can still achieve the crawl-ratio you want by using aftermarket low-range T-case gears.
Improvements to T-case gearing make a much bigger difference to low-range performance than an axle or tranny swap. Consider a stock Toyota truck with a 3.93:1 First gear, a 2.28:1 T-case and 4.10 gears--the crawl ratio is 36.74:1. Add only a trans with a 6.34:1 First and the crawl ratio is 59.27:1--and the benefit is only in First gear. Add only the lowest-available 5.71:1 axle gears and you'll crawl at 51.16:1--and the freeway rpm is intolerable. But add just a 4.7:1 T-case gearset from Marlin Automotive and the ratio soars to 75.73:1 with all other components stock--and there are no drawbacks on the street!
However, T-case gearing can pose problems. We've heard that too-low gearing can make stopping the truck nearly impossible with an auto trans. Juice trannies will also need a full-manual valvebody so you can start in Second or Third, and so it won't upshift instantly out of First. Even with a manual trans, one super-low T-case can actually be too low--it's nice if you can choose from one 'case or two and not use ultralow if you don't need it. And remember that extreme low gears are as likely to break parts as stall the engine if you're in a serious bind.
Swapping to larger tires often requires installing lower axle gears. How much bigger can you go without a gear change? About 5 percent depending on the engine, the existing axle ratio and how and where you drive. Here's a quick way to approximate the gearing for any size tire: For V8-powered trucks multiply the tire diameter by .12. For example, the 33s shown would require 3.96:1 gears (.12 x 33); 4.10:1 are the closest gears available. For smaller, higher revving engines multiply by .14.
It's not uncommon to find gears that are unmarked, especially in the transmission. To learn the ratio of any gearset, divide the number of teeth on the driver gear into the number of teeth on the driven gear. For example, if the ring gear (driven) has 41 teeth and the pinion gear (driver) has 11, the equation would look like this: 41/11 = 3.73. Remember to always divide the driven gear by the driver gear--not the larger number by the smaller number--because the gears' size relationship is reversed in an overdrive situation.
Piggyback transfer cases in one form or another are becoming increasingly popular because of the amazing final-drive ratios they afford in low range. An ordinary Toyota transfer case has a low-range reduction of 2.28:1. If you team Toy T-cases as done with the Marlin Crawler or Ice Crawler, the effective low-range ratio becomes 5.20:1 (2.28 x 2.28=5.198).
Not all ratios are available for all axles, so it's important to check if the ratio you plan to install is available for both the front and rear axles. This will be especially true if you swapped in a different axle at one end--but you always need the same gear ratios in both axles or the T-case will erupt. However, don't be put off by a variance of just a couple of hundredths. For example, the popular 4.10 gears may be called 4.09, 4.10 or 4.11.
To calculate the effects of an over- or underdrive unit on a final-drive ratio, the ratio of the accessory-drive unit is multiplied by the transfer-case ratio. For example, a truck with a .76 overdrive tranny, an .86 overdrive unit, a 1:1 high-range T-case and 3.55 axle gears has an ultimate final-drive of .76 x .86 x 3.55 = 2.32:1. Going in the other direction--crawl ratio--a TH400 with an aftermarket 3.00:1 First gear, an NP203 with 2.00:1 low-range ratio, a 1.37 underdrive unit and 4.88 axle gears would have a crawl ratio of 3.00 x 2.00 x 1.37 x 4.88 = 40:1.
GEARHEAD MATH
When considering the gearing ideas posed in this story, it'll help if you have some mathematical formulas for calculating. The following will help you find your engine rpm at any speed in any gear in either high range or low range:
(mph x axle ratio x trans ratio x T-case ratio x 336) divided by tire diameter = rpm
Therefore, a truck going 65 mph with 34.5-inch tires, 4.56 axle gears in Fifth gear (.72:1 overdrive) and high range on the T-case (1:1) would look like this:
(65 x 4.56 x .72 x 1 x 336) divided by 34.5 = 2078 rpm
By plugging in different gear ratios for the axles, tranny and T-case, you can determine the best combination of gears for your needs. For example, if the truck described above was a four-cylinder, 2078 rpm at 65 mph may not find the engine in its powerband. By swapping to 5.38 gears it would spin 2452, a bit closer to what's needed. Next, drop the tire size to 32.5 and the rpm climbs to 2603. If that same truck was downshifted into Fourth (1:1) at 65 mph, it would scream the motor to 3615 rpm.
The same formula can tell you how slow you can crawl. Here's what it looks like when applied to a Jeep going 3 mph with 5.13:1 axle gears, a 6.34:1 First gear, a super-low 4.0:1 low range and tires that have been aired down to 34 inches. It can go 3 mph at 3857 engine rpm! That's good.
(3 x 5.13 x 6.34 x 4 x 336) divided by 34 = 3857 rpm
Here's another version of the equation you can use if you prefer to solve for mph, since your speedo may not be accurate.
(rpm x tire diameter) divided by (axle ratio x trans ratio x T-case ratio x 336) = mph
And if you don't know the gear ratios in your manual tranny, you can figure them out using this formula, assuming you get accurate info from your tachometer and speedometer:
(rpm x tire diameter) divided by (mph x 336) = axle ratio x trans ratio x T-case ratio
When using these formulas, keep in mind that you have to measure the actual height of the tires on your truck at running pressure--don't trust the height number printed on the sidewall because it's always optimistic. Also, auto-trans-equipped trucks often have a little slip in the torque converter, which will cause rpm to be a bit higher than indicated.
CRAWL RATIO
The latest talk around the campfire is about super-low crawl ratios, or overall lowest-possible gear ratio for way-slow trail crawling. Here's how to figure it:
Trans ratio x T-case ratio x axle ratio = overall ratio
Example: 6.34 x 2.72 x 4.88 = 85.15:1
If you have two transfer cases, then you would include two T-case ratios in the formula. The formula can be used to calculate the overall gear ratio with any combination of gears in the transmission, transfer case and axles.
Crawl ratios in the neighborhood of 80:1 for manual transmissions are good, and anything over 100:1 is impressive. Automatics use a torque converter and gears to multiply torque, and the fluid cushion between the engine and trans lets an auto get away with higher crawl ratios; 30:1 is typical and over 40:1 is a lot. Tall tires, manual transmissions and small engines all demand a lower crawl ratio; automatics, short tires and big engines can reduce the need for a lower crawl ratio.
TALL-TIRE SHORT CUT
If you just want to restore the stock overall gearing after installing larger tires, here's a mathematical short cut:
new tire diameter divided by old axle ratio times old axle ratio = new axle ratio
Assuming a stock axle ratio of 3.73 and tire height of 31 inches, here's how to figure it:
36 divided by 31 times 3.73 = 4.33
The closest gears are 4.10 or 4.37, if available. Since tall tires are heavier than stock, we suggest the lower gears since you may need a slightly lower-than-stock ratio to maintain the acceleration you had when stock.
GEAR SWAPS
Bigger tires are on top of every must-have list. But larger tires make a gear change necessary. Gearing improvements can be done to the axles, transmission or transfer case. There are advantages to each depending on the results you're trying to achieve. Some people may be looking for the ultimate crawl ratio; others may just want to get the overall gearing back to where it was before the todious meats were installed. Then there are those who want to match a specific engine rpm to a certain road speed. Since everyone's needs and mechanical combinations are different, there's no magical chart yielding the perfect gearing. This story will tell you how to make the right choices for your 4x4. It will help if you read the "Gearhead Math" and "Crawl Ratio" sidebars first.
AXLE GEARS
We get a ton of questions about axle gearing versus tire size, so let's start with extreme basics: Adding taller tires or higher-ratio gears reduces engine rpm at all road speeds and gives the truck less mechanical advantage to help it crawl over an obstacle or get moving from a stop. Smaller tires or lower gears increase engine rpm and acceleration/ crawling capabilities. Low gears are higher numerically (4.10 is lower than 3.50) and high gears are lower numerically (3.50 is higher than 4.10).
Once you understand those basics, you'll see why adding taller tires can cause the engine to lug below its powerband and cause poor performance. To fix the problem, you need to swap to lower-ratio axle gears. The "Tall-Tire Short Cut" sidebar tells you how to regear for stock performance. But many 'wheelers want gears that make the overall combination even lower than stock to improve towing or off-highway prowess. Others may have highway-bound 4x4s that turn too much rpm on the freeway and need higher-ratio axle gears to lower the engine rpm. You can make small changes to the truck's performance with tire sizing: Shorter tires act like lower gears (more highway rpm) and taller tires act like higher gears (less highway rpm).
The good and bad news about axle-gear swaps and tire-size changes is that they affect performance in any trans gear and in either high range or low range. That's good because a simple swap is all it takes to restore performance to what it was before you put on tall tires. The bad news is that very low gears for dirt performance may cause more highway rpm than you can live with, and swapping to high gears for cruising at speed will hamper towing and off-highway ability. When balancing both ends of the scale, your gearing choices become fairly limited. That's why proper axle-gearing choices must be made with the transmission and T-case gears in mind. In many cases the cost of new gears might be as much as an overdrive tranny swap that would give you the best of both worlds. Read on...
TRANSMISSION RATIOS
Tranny swaps are for 'wheelers who want more tuned gear-ratio performance than is possible with axle-gear swaps. A trans swap can net either a very-low First gear or an overdrive--or in the case of the NV4500 trans, both. If you don't have an overdrive tranny and are happy with the highway rpm of your current axle gears, then a trans with a lower First-gear ratio can provide the low-speed trail performance (crawl ratio) you're looking for. Alternately, if you like your current low-geared axles for 'wheeling but hate the rpm on the freeway, then you'll want to swap to an overdrive tranny--usually an automatic with a .70:1 Fourth gear or a manual with a .75:1 Fifth--to drop engine rpm at high road speeds. High axle gears and a low First gear or low axle gears with an overdrive are different means to the same end.
Ready to spend more bucks for performance? Consider this: If you're happy with your freeway cruise rpm but want a lower low range, swap in an overdrive tranny and lower axle gears. If you calculate the ratios to achieve the same freeway rpm you had without overdrive, then the lower axle gears will give you a lower crawl ratio at no cost to highway speed. Many overdrive trannies also have a lower First gear, so you win on both ends. Do the math and you can come up with some really interesting combinations.
However, tranny swaps can have drawbacks. They won't cure the problem of tall tires and too-high axle gears because they usually have only a lower First gear, so while you may fix the stoplight acceleration problem, you still won't be able to hold speed on the highway. Also, just as with axle-ratio swaps, tranny changes affect performance whether you're in high range or low range. If you swapped in a granny-low four-speed you'll have to live with it on the street. If the trans isn't geared low enough in Second, or if your engine doesn't make enough power to pull away from a stoplight in Second, then you'll get really irritated having to row through the ultralow First. In some cases First may not have synchromesh.
You also have to consider how far rpm will fall between gears. Calculate your road speed and rpm in each gear or else you may end up with a Fourth gear that lugs the engine at your cruise rpm but a Third that runs at too much rpm. This is especially a problem with small engines, heavy vehicles and towing. Automatics are less of a worry than manuals on this score, but keep in mind that many four-speed autos should not be operated in overdrive for towing. And if you swap an aftermarket low First/Second gearset into an automatic, be careful that the rpm drop from Second to Third isn't too great.
TRANSFER CASE
The latest trend in gearing is to alter the low-range ratio of the transfer case by either changing internal ratios or mating boxes to double the low-range ratio (see "Learning to Crawl" elsewhere in this issue). This is a particularly interesting solution for 'wheelers who want the lowest gearing possible yet still drive on the street. Changing the low-range ratio won't alter the high-range performance (except in Samurais because high range is also lowered) so there are no on-highway compromises, but you have to sort out your axle ratios before you begin. A low T-case can omit your need for an overdrive tranny if you gear the axles for a perfect cruise rpm and can still achieve the crawl-ratio you want by using aftermarket low-range T-case gears.
Improvements to T-case gearing make a much bigger difference to low-range performance than an axle or tranny swap. Consider a stock Toyota truck with a 3.93:1 First gear, a 2.28:1 T-case and 4.10 gears--the crawl ratio is 36.74:1. Add only a trans with a 6.34:1 First and the crawl ratio is 59.27:1--and the benefit is only in First gear. Add only the lowest-available 5.71:1 axle gears and you'll crawl at 51.16:1--and the freeway rpm is intolerable. But add just a 4.7:1 T-case gearset from Marlin Automotive and the ratio soars to 75.73:1 with all other components stock--and there are no drawbacks on the street!
However, T-case gearing can pose problems. We've heard that too-low gearing can make stopping the truck nearly impossible with an auto trans. Juice trannies will also need a full-manual valvebody so you can start in Second or Third, and so it won't upshift instantly out of First. Even with a manual trans, one super-low T-case can actually be too low--it's nice if you can choose from one 'case or two and not use ultralow if you don't need it. And remember that extreme low gears are as likely to break parts as stall the engine if you're in a serious bind.
Swapping to larger tires often requires installing lower axle gears. How much bigger can you go without a gear change? About 5 percent depending on the engine, the existing axle ratio and how and where you drive. Here's a quick way to approximate the gearing for any size tire: For V8-powered trucks multiply the tire diameter by .12. For example, the 33s shown would require 3.96:1 gears (.12 x 33); 4.10:1 are the closest gears available. For smaller, higher revving engines multiply by .14.
It's not uncommon to find gears that are unmarked, especially in the transmission. To learn the ratio of any gearset, divide the number of teeth on the driver gear into the number of teeth on the driven gear. For example, if the ring gear (driven) has 41 teeth and the pinion gear (driver) has 11, the equation would look like this: 41/11 = 3.73. Remember to always divide the driven gear by the driver gear--not the larger number by the smaller number--because the gears' size relationship is reversed in an overdrive situation.
Piggyback transfer cases in one form or another are becoming increasingly popular because of the amazing final-drive ratios they afford in low range. An ordinary Toyota transfer case has a low-range reduction of 2.28:1. If you team Toy T-cases as done with the Marlin Crawler or Ice Crawler, the effective low-range ratio becomes 5.20:1 (2.28 x 2.28=5.198).
Not all ratios are available for all axles, so it's important to check if the ratio you plan to install is available for both the front and rear axles. This will be especially true if you swapped in a different axle at one end--but you always need the same gear ratios in both axles or the T-case will erupt. However, don't be put off by a variance of just a couple of hundredths. For example, the popular 4.10 gears may be called 4.09, 4.10 or 4.11.
To calculate the effects of an over- or underdrive unit on a final-drive ratio, the ratio of the accessory-drive unit is multiplied by the transfer-case ratio. For example, a truck with a .76 overdrive tranny, an .86 overdrive unit, a 1:1 high-range T-case and 3.55 axle gears has an ultimate final-drive of .76 x .86 x 3.55 = 2.32:1. Going in the other direction--crawl ratio--a TH400 with an aftermarket 3.00:1 First gear, an NP203 with 2.00:1 low-range ratio, a 1.37 underdrive unit and 4.88 axle gears would have a crawl ratio of 3.00 x 2.00 x 1.37 x 4.88 = 40:1.
GEARHEAD MATH
When considering the gearing ideas posed in this story, it'll help if you have some mathematical formulas for calculating. The following will help you find your engine rpm at any speed in any gear in either high range or low range:
(mph x axle ratio x trans ratio x T-case ratio x 336) divided by tire diameter = rpm
Therefore, a truck going 65 mph with 34.5-inch tires, 4.56 axle gears in Fifth gear (.72:1 overdrive) and high range on the T-case (1:1) would look like this:
(65 x 4.56 x .72 x 1 x 336) divided by 34.5 = 2078 rpm
By plugging in different gear ratios for the axles, tranny and T-case, you can determine the best combination of gears for your needs. For example, if the truck described above was a four-cylinder, 2078 rpm at 65 mph may not find the engine in its powerband. By swapping to 5.38 gears it would spin 2452, a bit closer to what's needed. Next, drop the tire size to 32.5 and the rpm climbs to 2603. If that same truck was downshifted into Fourth (1:1) at 65 mph, it would scream the motor to 3615 rpm.
The same formula can tell you how slow you can crawl. Here's what it looks like when applied to a Jeep going 3 mph with 5.13:1 axle gears, a 6.34:1 First gear, a super-low 4.0:1 low range and tires that have been aired down to 34 inches. It can go 3 mph at 3857 engine rpm! That's good.
(3 x 5.13 x 6.34 x 4 x 336) divided by 34 = 3857 rpm
Here's another version of the equation you can use if you prefer to solve for mph, since your speedo may not be accurate.
(rpm x tire diameter) divided by (axle ratio x trans ratio x T-case ratio x 336) = mph
And if you don't know the gear ratios in your manual tranny, you can figure them out using this formula, assuming you get accurate info from your tachometer and speedometer:
(rpm x tire diameter) divided by (mph x 336) = axle ratio x trans ratio x T-case ratio
When using these formulas, keep in mind that you have to measure the actual height of the tires on your truck at running pressure--don't trust the height number printed on the sidewall because it's always optimistic. Also, auto-trans-equipped trucks often have a little slip in the torque converter, which will cause rpm to be a bit higher than indicated.
CRAWL RATIO
The latest talk around the campfire is about super-low crawl ratios, or overall lowest-possible gear ratio for way-slow trail crawling. Here's how to figure it:
Trans ratio x T-case ratio x axle ratio = overall ratio
Example: 6.34 x 2.72 x 4.88 = 85.15:1
If you have two transfer cases, then you would include two T-case ratios in the formula. The formula can be used to calculate the overall gear ratio with any combination of gears in the transmission, transfer case and axles.
Crawl ratios in the neighborhood of 80:1 for manual transmissions are good, and anything over 100:1 is impressive. Automatics use a torque converter and gears to multiply torque, and the fluid cushion between the engine and trans lets an auto get away with higher crawl ratios; 30:1 is typical and over 40:1 is a lot. Tall tires, manual transmissions and small engines all demand a lower crawl ratio; automatics, short tires and big engines can reduce the need for a lower crawl ratio.
TALL-TIRE SHORT CUT
If you just want to restore the stock overall gearing after installing larger tires, here's a mathematical short cut:
new tire diameter divided by old axle ratio times old axle ratio = new axle ratio
Assuming a stock axle ratio of 3.73 and tire height of 31 inches, here's how to figure it:
36 divided by 31 times 3.73 = 4.33
The closest gears are 4.10 or 4.37, if available. Since tall tires are heavier than stock, we suggest the lower gears since you may need a slightly lower-than-stock ratio to maintain the acceleration you had when stock.
Cue-Ball
03-10-2002, 10:47 PM
Yeah, too bad our G20's aren't RWD/4WD with a solid rear axle and transfer case, huh? :rolleyes:
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