Bigger Tires

[2000 YJ]

My 2000 TJ was purchased two years ago. It had 30" tires on it. I had a 4" lift, and added 33" tires. Now I have noticed that when I am on the highway, I can't go faster than 65 mph, and lose speed in 5th gear. Is this due to the bigger tires, or another problem. The Jeep has 37000 miles on it, and no history of engine or transmission problems. It is a 4cyl, I know, why do this to a 4, but it looks cool. Any help is appreciated.

[Ray H]

Originally Posted by 2000 YJ

My 2000 TJ was purchased two years ago. It had 30" tires on it. I had a 4" lift, and added 33" tires. Now I have noticed that when I am on the highway, I can't go faster than 65 mph, and lose speed in 5th gear. Is this due to the bigger tires, or another problem. The Jeep has 37000 miles on it, and no history of engine or transmission problems. It is a 4cyl, I know, why do this to a 4, but it looks cool. Any help is appreciated.

Its due to a couple things.
1) you need more mechanical advantage (ie:lower gearing)
2) you are no longer in the powerband (RPM range) that the engine is designed to be strongest in.
3) those 33s weigh alot more than the stock 30s so even if you were at the proper RPM, you would still notice a power drop because it takes more power to turn the heavier tires. You should also have noticed that it doesnt stop as well as it did also. Once those heavty tires start moving, it takes more to stop them.
4) Unless youve changed speedo gears, you are actually traveling faster then your speedometer says you are (example: you say you cant go faster than 65, when you actually cant go faster than 75).

[Kuhn]

OK... I'm going to state the obvious. It is a 4cyl... and there is nothing wrong with that; however, you will need that "mechanical" advantage, meaning gearing. I'm not sure what gears would be best, but if you go for a 4.11 you are risking horrible gas mileage (high rpms) with 33's at freeway speeds.
I have nearly the same problem with my 4.0L and 35's. I am going to try 4.56 gearing soon, so I will start a thread when I do. I'll let you know whether it turned it into an off-road monster without the possibility of driving over 55mph.
Just remember that the 2.5L 4cyl has just as much potential offroad, but that will kill the on-road ability.
If you have the funds... turbo your 4cyl. That gives you the high end power. If you want to off-road, gearing is the answer. That's my

[scoobytuff]

Yeah, the 33" tires kinda kill road performance. A set of 4.56 gears might not be a bad idea. When you say 65mph, is tha the corrected number for the new tire size? Also, the higher your Jeep sits off of the ground, the more wind resistance your are getting. This plays a smaller factor than being at a mechanical disadvantage, but all of that stuff adds up quick. The turbo idea is cool but expensive if you buy a kit. I did see a Jeep on dragtimes.com that a guy had a home brew turbo on. It was mounted remote ( where the muffler would be) and did not have an intercooler. The turbo had only like 4.5 pounds of boost and came out of a Volvo 850 (from a junkyard). With a 30" tire, and nothing besides somekind of computer tuner he managed sub 14 second quarter mile times. Impressive!

[jeep mechanic]

Before you guys go to 4.56 check out superior gear they have a 4.88 that was designed just for the four banger. i have a 1997 2.5 TJ , 4.88 gears, 33-12.50 tires. 4.5 lift. i didnt have to change speedo. its reads correct! and it will still run well in 5th gear. i believe in 5th gear at 65/70 mph 2900 to 3000 rpms. top speed is 85mph but it sounds like the driveline is gonna come loose at 80. check out the 4.88 first. I did and it works great