How to correct computer if smaller sidewall tires are put on?

[yellowcavalier13]

Hi I have a 2003 cavalier ls, The tire size that is on it is 195-70r-14 I am thinking about getting tires which are 175-70r-14 because I was told that the smaller sidewall size would give better bottom end performance (0-40, 0-60, Which is what i'm looking for) However He also said that it would alter my speedometer rating, and the amount of mileage that is put on the car, That it would read that there are more miles on the car that I would actually be putting on.

Is there a way to correct the sensor or device which reads the tire revolutions?

Thanks,

YellowCavalier13

[notaricajustfasta]

well....no it wouldnt affect the speedometer rating because you still have 14 inch rims with a 70 profile on it. and it wouldnt affect what the odometer reads either. plus the only it might affect performance is by making the tires lighter. plus you will have more understeer (or oversteer) i cant remember which, becuase of the skinnier tires. who did you talk to? please dont ask them anymore about tires on your car.

[jakegday]

just thinking here, would that be a performance negative, since you'd have less traction?? less tire on the ground....

[Derk_eliso]

actually... with 70's... the same tire is the same height so it doesn't affect the speedo...

less tread on ground = less traction.... it's oversteer... you wanna try to drift or grip?

[yellowcavalier13]

I'm just looking to get the best low end speed (0-40 speed)
And to my understanding tires are an important part.

[Mangoamerican]

so anybody know how to do it?? in my mind... smaller tires = more revolutions per minute which would make the car wind out more during acceleration - because it doesnt have to work as hard, but it wont be as fast... just get there quicker - maybe all those lowrider guyz did that for a reason check out this article if you dont get an answer - id be surprised if you do, seems all the guys that know anything in here are out to lunch http://www.tirerack.com/tires/tirete...&currentpage=5

[jakegday]

if you just want a great 0-40 time, you'll want as much traction as possible, (takeoff is critical in that short of a distance) so you would not want to get these smaller/thinner tires

[Classicrocjunkie]

newbs.......



Just because your side wall is staying at 70, doesn't mean your speedo will stay true. Like i'd said it before and i'll say it again. Use the proper sized tires for your vehicle closest to being true. Doing something half assed will leave you in the end with a half assed working car...

[pchanson42]

Hey, I don't know how to correct the computer, but the others who answered didn't answer the tire size information correctly IMO. First of all, a shorter sidewall will provide more low end capabilities just as much as changing final drive gear ratio does. Simple math and physics. I'm not sure exactly what computer settings are affected by a change in tire size but I would imagine the speed at which the lock-up TQ converter kicks in would be one. If you have a manual trans, then that becomes moot. Then, by my thinking, you only need to adjust speedo. That can be changed with a speedo pinion gear swap. A speedo gear chart can help with this. There are several out there where you put in final drive gear ratio, speedo drive gear (not the driven gear as that is the one you would be changing) and the tire size. To find out the drive gear size, you need to look at where the speedo cable hooks to the tranny and remove that and the cover/assembly and driven gear. The dive gear will then be visible and is most likely color coded. There are also places on the web that will translate that color to a tooth count and once you know that, you can order the correct driven gear.

Tire sidewall height I.E "65, 70, 75" is an expression as a percentage of width I.E 215, or 205. So if you go wider width and keep the percentage the same, 70% of 215 is GREATER THAN 70% of 205 and therefore your overall tire size will be taller. I think the OP already knew this. This info is for the ones who answered incorrectly.

The formula for overall tire diameter is: width x (times) series divided by 25.4 (mm to inches) x2 (for the rubber under the rim and the rubber over the rim or in other words, you cross 2 sections of rubber plus the rim when you lay a tape measure across the entire rim/tire assembly) + rim diameter.

In your case: Current size= 195-70r-14 195 x .70 = 136.5 mm sidewall height. divided by 25.4 = 5.374 inches x2 = 10.75 (rounded up) + 14 = 24.75 or 24 and 3/4 in tire height. Go measure and check my math with the tire off the car or at least properly inflated but even then it will be squished down some.

For the tire you want to switch to: 175-70r-14 The overall diameter of that tire = 24 inches. So, yes, you are correct, it is smaller sidewall and will provide more low end torque to the road, but it is a narrower tire so it will also have less traction (which someones else accurately pointed out).

To resolve this, you can keep the same width tire and run say a 65 series sidewall ratio. P195/65R14 if you can find one. That will yield a 24.34 in tall tire. About half inch smaller than current.

Another option is to go up 1 size in width to P205 and drop 2 sizes in aspect ratio. Make sure that tire will fit the rim (there are charts for that and also you should be able to find factory rim widths for your car and year). A P205/60R14 will give you a tire height of 24.04 inches. Almost exactly the same as the tire you wanted to switch to with 15% more rubber on the road... If it clears the tie rods and inner/outer fenders and any other misc. crap that may get in the way, you're golden...

This is what I'd do right here on my 94 2.2 cav ^^^^^^^^ if I had an auto trans and will be doing to my 307 4bbl Olds powered 84 Buick Riviera (not those sizes I have 15 in rims and my current tire size= P215 which is already up one size from stock so I will only be changing aspect ratio). It is also a front wheel drive. v-8 four barrel with front wheel drive with all that v-8 weight out over the front tires for extra traction,, how cool is that?? I too will also be seeking info on how all that effects the computer and lock up TQ converter and also why this tire size stuff is fresh in my mind, because I've been researching the exact same thing.

Hope that helps,

Paul

[pchanson42]

Holy resurrected thread batman!! I should have checked the date before replying. Still, what I posted might help someone in tire size selection, so all is not lost.

Paul