High RPM Poster
Feb 8, 2009
Greg in Toronto/Gtown
1999 C5 Z51
So I am in the process of getting some new tires for the car and was trying to figure out how to spec in the right sizes
for the front and the rear.
If your tire diameter exceeds the 5% different in rotation from the front to the rear then the AH/TC will kick in from what
I have read as it would think you are loosing traction.
I will be putting the tires on the Z06 style repro rims but they have the same stock width size of the OEM rims.
I currently have Firehawk SZ50 EP Run Flats, 245/45R17's and 275/40R18's and was thinking of going with the Kumho
Ecsta MX's or the Nitto Invo's.
Below I tried to figure out the tire diameters and difference from the front to rear and the percentages.

The Firehawk's specs are currently 245/45R17=25.7" diameter and the 275/40R18=26.6" diameter.
If we calculate the overall diameter difference then we take 26.6"-25.7"=0.9"div by 25.7"=0.035x100%=3.5%
There is only a 1.5% threshold for the AH/TC to kick in at this point, is it better to get the % closer to zero as possible?

If I take the Kumho Ecsta MX specs are 255/45ZR17=26.1" diameter and the 295/35ZR18 = 26.1"diameter
then these are a perfect match with no difference in the overall diameter so the AH/TC would not kick in premature.

If I take the Nitto Invo specs 245/45/17=25.79" diameter and the 295/35/18=26.02" diameter.
If we calculate the overall diameter difference then we take 26.02"-25.79"=0.23" div by 25.79"=0.0089x100%=0.89%
There is a 0.89% difference in the two tire sizes.

Does this make sense on the way I have written this up and will the % difference play a big roll with the AH/TC kicking in?

What do you think is better the Nitto Invo's or the Kumho Ecsta MX's for the money? The Kumos are about $1150 installed
and the Nittos would run me around $1350 installed.

Any input on tire size and AH/TC issues would be appreciated.
I may be wrong, but from what I have read, the rear tires should be at least a half inch larger in diameter to satisfy the TC/AH. Either that, or you can just drive in competition mode all the time.:rofl:

If I was choosing between the two tires, I would have to go with the Nitto's.
step 1

step 2

i'm pretty sure you adjust the VSS Tire Revs per but i haven't done it and wouldn't want to recommend anything.
talk with a tuner about it and they could advise.
sorry i couldn't get you further...
Last edited:
Hp Tuners will only edit the speedo, not the AH or ABS. The rear tires need to be ~1" taller than the front tires for the AH & ABS to function correctly, and so that the height of the front & rear tire sidewalls match.

It looks very odd when the front sidewalls are taller than the rear.

Russ Kemp

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