This is for my rear wide body base car on 19s.

Currently running 275/30/19 (25.6)with rear 345/30/19 (27.2)
With this setup I don't have any issues. When I put my 17s on the back with a 28 it's a hit or miss with the stability and abs sensors.

Looking for input on switching from 275/30 (25.6) to an 275/35 (26.6) with the same rear 345/30 (27.2).

Is it now too close of a ratio moving up in size? Also I'm trying to put r888rs on all the way around and this is the only sizes I can get them in.
If I do different tires up front I have the option of 25.6 or 26.6.

Thanks for the help
Pfa

 

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.

 

The ideal (factory) front to rear ratio is: front diameter x 1.04 = rear diameter.

Last year we had an interesting discussion with members who have done combinations outside that ratio, some had problems while others didn't. Based on those conversations, in my opinion, the outside limit is probably front diameter x 1.08. We don't know how close you can go to 1:1, that would be a new experiment. If I were a guessing person, and I'm not, I would guess probably not less than 1.03, maybe 1.02.

Obviously all cars and computers are not "exactly" equal so you might have a problem at 1.06 or 1.07. So experiment with what you can get your hands on and see how far "your car" can go.

Good luck and please let us know how you do. We can add your experience to the knowledge base.

 

I ran 25.6 and 27.2 with no issues. That would be a 1.065 ish I believe. I went ahead and stayed with what I know works. I was going to try a 26.6 and 27.2 but not going to chance it.