: Different front/rear tire diameters



WA 2 FST
08-31-12, 12:27 AM
So what is the real truth about this? I hear that there is a certain limit as to how much different these can be (front to back) without messing up with the TC/ABS controls.

But then I hear no one _really_ knows what the limit is. Can this issue be alleviated with tuning?

I mean, guys who are running the 305/35-19 MT drag radials, are now running a rear tire that is .6" taller than stock (and .35" taller than the front tire diameter). I don't hear of any issues.

Anyone have the real story here? I'm contemplating a combination that will have 1.0-1.2" difference in diameter (taller in rear).

Tedboss1
08-31-12, 09:34 AM
I'm running the 305/35-19 MT drag radials and I have a problem with them.
Around 50 MPH with light throttle applied my car starts bucking so I have to turn the TC off (one click) and the problem goes away.
Some people have reported same problems others don't seem have it at all.
Provably has to do with different version of software in my car.
My car was build late in 2008.

WA 2 FST
08-31-12, 10:12 AM
Tedboss... _thank you_ for the timely and informative response. The good thing is that you can turn it off, and there is no override from the PCM. I never drive my car with TC on, as I run in Comp Mode (two clicks) all the time. Maybe even if mine acts up, I can just be sure TC is disabled.

Havasusteve
09-01-12, 10:42 AM
The key to any significant change in tire/rim diameter is changing the ECU parameters. I run a 305.30.19 normally, 305.35.18 for DR at the track with no issues. In my case, nothing has been changed, but I am aware that 20 rims or 17" rims for drag purposes can alter things.

tedcmiller
09-01-12, 01:53 PM
All I know is that different sizes front-to-rear means that you can't rotate tires front-to-rear. Side-to-side is OK provided that the tires are not directional. Changing the wheel diameter will probably also screw up the speedometer, unless the wheel rotation per mile is kept constant by choosing tires with appropriate aspect ratios so that the wheel rotation/mile is essentially unchanged.

WA 2 FST
09-01-12, 02:46 PM
All I know is that different sizes front-to-rear means that you can't rotate tires front-to-rear. Side-to-side is OK provided that the tires are not directional. Changing the wheel diameter will probably also screw up the speedometer, unless the wheel rotation per mile is kept constant by choosing tires with appropriate aspect ratios so that the wheel rotation/mile is essentially unchanged.

Well, we can't rotate the tires front to rear as it is. As far as messing up the speedo calibration... I agree, it does do that, but should be easily tuned in the ECM to reflect the different rear tire diameter to show accurate revs/mile.