So.. you're all welcome to roast me, but please also lend me some helpful info. Ill admit it: I didn't do enough research. I recently installed a set of M&H Racemasters on my car and have now put the car out of its parameters. Current setup: 11 Grandsport - Weld RTS s71's all around - 17x8 up front 17x11 rear. Front tires: 245/45/17 Nittos, and rears are 325/45/17 M&H. (previously had a 315/35 on the rear) I researched the tire and saw a ton of people using it, didn't think it would put me out of specs. Have the xmas lights on (abs/brake/service traction control and active handling etc etc). Do I have to go 18" up front? Can I get away with taller front tires? I've been searching but haven't found anything to give me a solid answer. I can throw the front stocks on for now and I think that will put it back within its parameters but I need a permanent solution for this. Thanks for any help. edited to add some calculations: Front is 4.34" sidewall- 25.7" diameter. Rear setup is now a 5.75" sidewall - 28.5" diameter - so there is a 1.41" difference between the front and rear sidewalls and the overall diameter difference is 2.8" . What is the maximum amount allowed?

 

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.

 

Thanks for the information! I'm sure you can help me but you seem more keen on having me work the math out myself. I am looking for someone with experience on this issue to help.. someone whos worked their way around it on a gs/z06 first hand. I don't have the means to retrieve all that information at the moment, the car is not with me. I have the numbers off the current wheel and tire setups so if anyone knows the actual parameters I need to be within or have actual experience and can tell me what I should be running up front to avoid the nannies I would be extremely grateful. I can't find any solid information. One thread says one thing, another says something else. Someone even wrote that they were running the stock GS wheel and tires up front with my exact rear wheel and tire combo with no problems. It doesn't make sense as the stock diameter is only a hair over 26" but I threw the fronts on today as I had to do front pads anyway, the problem obviously persists.

 

Just to help a little. Assuming you're not employing totally worn out tires on one end and brand new ones on the other. GS stock sizes are 26.7" diameter rear and 25.6 front. So rears are 1.1" taller than fronts....which means little. You need to be concerned with the front to rear ratio, not the difference in inches. So 25.6/26.7=.9588. If you want to use your 28.5" tall rears you'd need 28.5X.9588 or 27.3" fronts. If you want to use your 25.7" fronts, you'd use the stock rear height (26.7) divided by the stock front height (25.6) times the height of the front tires you're wanting to use. So 26.7/25.6X25.7=26.8. Yeah, you're mismatched pretty bad with 28.5 on the rear and 25.7 fronts.

Tirerack.com has tires sizes and diameter of the sizes listed for just about any tire made.

 

Thank you brother! Ok this is the information I needed. So I was using http://www.rimsntires.com/specspro.jsp for my calculations and it looks like if I put a 245/55/17 on the front it will bring the diameter to 27.6" and maybe ill be ok? Do the rears have to be 1.1" taller minimally or can I get away with these wheels putting it at a .9" difference? I found a Michelin (Primacy 3) tire at discounttiredirect.com but it's definitley an odd size for a performance tire (tagged as performance/summer) as 90% of everything i'm seeing is a run flat/winter tire at those specs. edit: Just playing around with that site.. if I attempt a 255/55/17 that brings it to 28" even.

 

Yes, I am. Because once you understand the limitations, you'll be able to calculate any tire combination and at any point during the tread wear. Obviously, you've already run into conflicting info, so why continue to ask others for opinions. It's simple 5th grade math and tirerack has all the specs you need.