JackLS1R1

I have 305/30/20 rear tires on the back of my C6. My tuner has the car tuned to this tire size (I believe this is programed to the TCM).

I am looking to be able to swap to rears with 275/40/17 for the track.

27.2" tire vs 25.7" tire

How bad would the shift points be off if I put on the 17" drag radials? Will I be hitting the limiter or maybe short shifting?

Thanks for any replies that are on topic

(No I am not asking an ABS or stabilitrak question!)

Russ K

Shift RPM won't change, just your speedo will read 5.8% high.

Russ Kemp

JackLS1R1

Are you sure? I just finally got a hold of my tuner, and he said there would be an issue?

I did some research and there are 2 criteria for a shift.... MPH and RPM... RPM wont change, but with different tire diameter, MPH would?

tblu92

Yes you are looking at an approx. -6% in final drive ratio with the smaller diameter rear tire---You are correct that in order to command a shift at WOT you need to hit 2 parameters--1 being MPH and the other RPM----With a smaller rear you risk that what they call the "SPLIT" in RPM at shift will become too close and the TCM will skip over the shift and hit the safety rev limiter-----
I would try it out 1st on the street to see if it shifts correctly--If it skips over the shift point then I would make the shift point correction in the "shift at WOT MPH tables "--at least by the 6%----The more modded and the more HP above stock your engine is the greater the "split" has to be----On a bone stock engine you can get by with a low 600 RPM split---But with added HP---Lower rear gear ratios-smaller rear tires--or a stall converter on an auto trans I have had to tune sometimes a 1000 RPM split in order for it NOT to skip over the WOT shift