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Discussion Starter · #1 ·
I see some of you have been running 17's on the rears at the track and it gives you better 1/4 mi times. But how does that affect T/C? Do you turn it off instead of running in Comp mode?
 

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Some knowledgable road racers also use 17" in the rear.

In fact I recommend:
305/30x18 on 10.5x18 Front
315/35x17 on 11x17 Rear

Some points to consider:

1. The TC & AH look for a wheel speed(s) that is approximately 5% greater (different) for activation. Wheel size difference may contribute to some of that % difference. Actual tire slip accounts for the remainder.

2. Most racers run with TC & AH OFF except in the wet.

3. It is the TIRE diameter and thus circumference that is important when making the comparison and calculating the size differnce contribution to that 5% difference, NOT the wheel diameter.

In my above recommendation for example, both the front and rear tires are 25.6 " diameter (notice that the aspect ratio accomplishes the tire equality of the different wheel sizes) and thus the AH & TC computer sees 0% size difference. It thus has a full 5% of tire slip to use in racing conditions before activation.

4. Race tires are much more "grippier" than street tires and this fact will sometimes cause problems with the TC & AH more so than a size or wheel speed difference. ie. the Computer sees that percentage change over a much shorter time. This phenomina also effects the ABS and can sometimes activate the dreaded "ICE Mode" in the ABS.

5. This 5% difference activation value can be reprogrammed, however 5% has been established by manufactureres and race teams using Pi & Motec systems and by F1 teams as the universal figure that most represents real world driving and racing conditions. It's almost considered a given constant

In summary, when selecting race wheels and tires you must consider the overall diameter of the combination. You DO NOT want a size difference that will result in more than a 5% wheel speed sensor difference. Anything LESS than 5% down to 0% (the less the better) is OK.

The real answer to your question is: It depends on the tire diameter and resulting circumference as compared to the front.
 

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Discussion Starter · #3 ·
Thanks for the detailed answer. I think you answered my underlying question when you said:

In fact I recommend:
305/30x18 on 10.5x18 Front
315/35x17 on 11x17 Rear

I was wondering if I could just swap the fronts with the rears at the drag strip, but I didn't want to sound like an idiot. :blush: I've read a few threads on tire size and T/C and I was wondering why, if it's just looking for a difference in size, couldn't I just swap the fronts with the rears and keep T/C happy?
 
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