nch209

Given stock base C6 tire ratio difference is about 1.044, how far can I deviate. Have front pilot super sports on the way in stock size, and using stock size super sports in the back yields basically the same 1.04 ratio (according to tire rack specs).

Say I want to go to a 295/35/19, that yields a ratio of about 1.054. Will the computer be ok, or will the nannies get all worked up?

Assuming the ratio works, should I be looking to +1 upsize the fronts too--- for handling reasons- to a 255 40 18

How far away- up or down- can the ratio be away from the stock 1.04?

Thanks MUCH!
-n8

PS. This tire stuff is confusing. Looking at tire specs is crazy- take michelin---- even a pilot super sport in the same market size has variants in it. Some made for gm, bmw, Mercedes with the same size are off quite a bit. A call to tire rack said Michelin tweaks them to what car markers want---- but it makes it confusing when shopping, and forget about used ! Lol! Print the spec sheet and see for yourself!

HOXXOH

Here's how to do the calculations, but you're going to have to do the math.

To determine the safe limits of deviation from stock size tires, that will not be affected by the nannies, do the following calculations.

First find the diameters and tread depth of both front and rear factory installed OEM tires.

For each tire, calculate the original (new) circumference and the circumference when the tread depth is zero. AKA bald.

Compare the front tire new circumference to the rear tire bald circumference to create a front to rear ratio.

Compare the front tire bald circumference to the rear tire new circumference to create another front to rear ratio.

Those two ratios are the safe limits. They may not be the ultimate limits, but they are safe.

Now simply do the same calculations for the tires you want to use. If they fall within the same ratio limitations as the OEM tires, then they'll work fine.

Since you're asking the question, it's most likely that the limits have been exceeded by one of the ratios. If the ratios are beyond the limits when both front and rear tires are new, you immediately run the risk of activating one of the nannies. If the new tire ratios are within the limits, but are near one end or the other, you can use the ratio limits to determine how much differential in front to rear tread wear will still fit in the acceptable range.

Assuming the new tire ratio fits the range, the chance of falling out of range is slim if the tires wear evenly. The problems may occur if one axle wears quickly and/or only one axle has replacement tires at a time.

This method will work to create the safe ratio limits regardless of the brand or sizes that were OEM.

nch209

^

Thanks for the explanation! Gotta crunch some numbers to see what is what, but was hoping there was a bit more leeway. Numbers don't lie, so off to calculate!

Thanks
Nate

cclive

This might be a good question for Tadge. It would be helpful to know, from the horse's mouth, what the actual ratio limit is.

nch209

Paging Tadge, paging Tadge!

Is he our resident expert???

cclive

As Chief Engineer for the Corvette...yes, he is.

nch209

Gotcha. How do I get him to read this! Sure he knows, but even more sure he's probably got bigger stuff on his plate than my question!

dustbusterplus

For Reference, my base coupe has 265/35/18 and 305/30/19 tires and my TC system acts up around 115-120. I'm guessing it starts seeing the difference in circumference at that speed. If I turn off TC and leave AH it doesn't try to slow me down at those speeds.

Kent1999

Easy way: most tires have a tread depth of 8/32 when new. Given that a tire is worn out at 2/32, that leaves 6/32 of useable tread. Take that times 2 gives the total reduction of diameter between old and new of 12/32, or about 0.6". So if a new tire is 26.0" a worn out one is 25.4", the engineers would certainly leave at least that much margin in the ratio to avoid unnecessary warranty service visits from cars that simply had new tires on front and well used tires on the back (or vice versa).

Bottom line for me is that means while the stock differential is about 1.0" to 1.1" (depending on model) we should be GTG with between 0.8" and 1.4" differential -- probably more, but we'll err on the side of caution. My last C6 I had a 1.5" differential and never saw an issue, but I didn't spend a lot of time above 120mph either.

This also means the further you stray from the 1.1" standard for new tires, the less your car will tolerate having tires that are worn unevenly front-to-rear.

cclive

He has enough on his plate just trying to stop the C7 from blowing up...

C6 4 AL

I have the same size tires you have no problem with TC at Auto Club Speedway doing 150 on front straight.

Mike's LS3

The oem GY EMT's have a 1.2" front/rear difference.
My Michelin oem size PS2 ZP's have exactly 1" difference diameter larger in the back and no issues. My next tire set up with wider rims to accommodate will be exactly your size in a Michelin Super Sport (non run flat) which also has the same 1" difference in diameter.

What is the tire diameter difference front and back? Which rims are you using?

Bill Dearborn

I use an ancient rule of thumb which is there is about a 4% variation in ratio where the systems will still work. The examples when used with the EMT tire diameters and tread depth give you a ratio of 1.053 if you have new rear tires and worn out front tires (2/32 tread remaining). The ratio with new front tires and worn out rear tires is 1.033. Those ratios are well within the 4% range. To be on the safe side just in case the systems have changed their thresholds over the years use a 3% ratio variance and you can probably get by with tires that provide a ratio of 1.014 to 1.074.

The C5 ratio is the same as the C6. I used to run equal diameter front and rear track tires (a 4% variation) on my C5s and never had an issue with traction control. Some individuals would have a problem at speed but I never did. I could run all day long with the TC on and it wouldn't intervene unless there was some actual wheel slip. I sort of think it had something to do with the slight variation in tolerances that can occur in the analog circuitry in the EBCM so some EBCMs would object and others wouldn't.

Of course the C6 is a little different with its engine drag control which means the rear tires can't grow in reference to the fronts.

Bill

dustbusterplus

I'm not sure of the exact diameter, but I'm currently running worn out Michelin PS2s on Ace slick 18x9.5 wheels in front and 19x11 in the back. My tires are near the wear bars on the rear, and I plan to get some Bridgestone S04s going into Spring and can verify whether or not it still occurs then.

Mike's LS3

Tire Rack lists Michelin PS2 (non run flats) with 25.3 front and 26.3 rear diameters which is 1" difference for your 265/35/18 and 305/30/19 tires. That should be fine. I'm surprised you have TC issues. I am on my second set of PS2 ZP's with 1" diameter difference and have not had any TC issues for a total of 53k miles. We both have the same diameter difference front and back. Also, your rims are the correct size for your tires.

I am curious to know what is causing TC to slow you done at those high speeds?

cclive

Tire Rack lists revs/mile for Michelin tires on their site...I think this is a more accurate measure to use in calculating the ratio than diameter. Also, normally if TC is going to have a problem, it is at exhibited at higher speeds. Many here have never had the C6 above 100mph, so the problem may never be seen.

dustbusterplus

The first time it happened I thought it might be the road I was on, but on a much better road it happened again. Whereas, if I turn TC off and keep AH on it never happens at similar speeds, it's only when everything is on. I never really looked into as I just thought it might be because of the different size tires.