MarkDFW

On my 2012 grand sport, the stock front overall tire diameter is 25.6 and rear is 26.7 per Tire Rack's site. I am told the traction control looks at the rotational relationship between the front and rear tires and if the rear gets too short relative to the front, the rear tire will be turning too fast relative to the front and the computer sees that as wheel spin and will create active handling issues.

25.6/26.7 = front/rear ratio of .959. So the question is what ratio is too high or too low such that AH will kick in? I've got a list of tires I can use at the track but want to make sure I stay within acceptable ratios so I don't get any surprises on the track!

FYI I am going from 18 front 19 rear to dedicated track wheels 18" all the way around.

JerryTX

I don't know about all that active handling mumbo jumbo since I run with it all off but I will say you are welcome to try my extra set of wheels on your car for 1 session.. they'll look goofy since your widebody but they'll stick!

MarkDFW

Thanks Jerry! I always put it in competition mode on the track but will be driving to and from the track when I get my new wheels/tires and would prefer to leave all assists on if possible. I've got a spreadsheet I built out of tires I can run and I think everything is close enough to stock diameter ratio. I have yet to find anywhere in manual or on the forum that states what an acceptable ratio is.

Here is what I have in spreadsheet. You've got load rating, speed rating, front size, rim width front, rear size, rid width rear, tread depth, wear rating, overall diameter front, overall diameter rear, and ratio F/R. Everything below should be fine for 18x11 front and 18x12 rear wheel setup, except for the Nitto NT555r2. Not sure if that ratio is too high?

GY F1 OEM Tire Characteristics LL Z 275/35R18 325/30-19 0.25 220 25.6 26.7 0.959

Continential Racing Slicks LL Z 285/645R18 10 to 12 325/680R18 11 to 13 25.5 26.8 0.951

Michelin Slicks Z 27/65-18 9.5 to 11.5 30/68-18 11.5 to 12.5 25.5 26.8 0.951

Pirelli Slicks Z 305/645-18 10.5 to 11.5 315/675-18 11 to 12

Hoosier R6 LL Z 275/35-18 9 to 11 345/35-18 11 to 13 0.125 40 25.5 26.8 0.951492537

Hooiser R6 LL Z 295/30-18 9.5 to 11 345/35-18 11 to 13 0.125 40 25.3 26.8 0.944029851

Hoosier R6 LL Z 315/30-18 11 to 12 345/35-18 11 to 13 0.125 40 25.6 26.8 0.955223881

Kuhmo Ecsta V710 Z 305/30-18 10.5 to 11.5 345/35-18 11.5 to 13.5 0.125 30 25.5 26.8 0.951492537

Nitto NT555R2 Z 285/35-18 9.5 to 11 305/35-18 10 to 12 0.15625 100 25.9 26.3 0.984790875

Pirelli slick rid width ranges are an estimate, since their site doesn't show rim ranges, but what I have above will fit 18x11 front and 18x12 rear. I also have tabs in the spreadsheet showing tire options with OEM wheels for GS/Z06. If anyone wants that spreadsheet PM me and include your personal e-mail. Jim Luther sent me his spreadsheet and I made modifications to it.

froggy47

The "acceptable ratios" seem to be:

a) somewhat of a secret as I have never seen official published information only guesstimates from other forum members/sources

b) seem to vary from year to year as gm & the gov't diddle with "safety concerns"

c) competition mode seems to widen the "acceptable ratio"

Right now I have shorter rear tires on just to knock around the street (got them free). The rears are .7 inch shorter. I get "traction system active" when I stomp on it, On my 04 z06 it dials the power back about 20-30%, that's all. If I put the car in comp mode OR all nannies off it goes away.

If you are going to track with nannies on (not recommended) I would definately make sure the rear is 1/2 to 1 inch taller and you "should" be ok. But mines an 04 and yours is a 2012 so who knows what the engineers did on your system?

MarkDFW

From what I can tell competition mode kills traction control and AH only kicks in if you get too far off. So far I've had no issues in competition mode on the track.

froggy47

This is likely the case, that comp mode turns tc off 100%.

dbratten

To add to the confusion, the listed diameter rarely calculates out to be the same as revolutions per mile (which I use thinking it may be more accurate) when I enter values into the computer. I also run with nannies off so it's not really a problem on track and a reset on the street takes care of the warning.

--Dan

Bill Dearborn

Tire diameter issues don't activate Active Handling. The differences in wheel speeds might affect how it works (brake application) once activated but that would be hard to ascertain. Tire diameter definitely affects traction control. I don't think C5s were as susceptible as C6s and I am pretty sure it varies somewhat on a car to car basis. For instance, my two C5s ran equal diameter front and rear tires and never had TC issues while some other owners did have issues using the same combination.

The C6 is more affected due to the drag control feature which actually speeds up the engine if the rear wheels are turning too slowly. Try to set your new tire diameters according to a ratio Vs a half inch or other type of measurement. As long as you keep the ratio of front to rear tire diameters within a couple of percentage points of each other the car will not be affected by a change in diameter from one set of 4 tires to another. The EBCM is programmed to expect a certain ratio in wheel speeds between front and rear and as long as you are close to that ratio you will be fine.

Bill