I think that 3% rule is for tires mounted to same axel, and full time AWD cars. If there is much difference in circumference between two tires on same axel, differential will not tolerate constant difference in speed of rotation between both tires over life of tire. It is only deigned for when car turns, not full time rotation correction straight down the road.
That being said, you can get away with miss-matched circumferences for a short trip of say 50 miles, as in doughnut spare tire use.
One of the rear tires of our AWD 300C was molested (2â€cut in side wall) by my wife when she backed into one of the concrete 12†tall walls between doors before the garage was built during house con$truction. I had only used about one third of tread, and tire place wanted to install four new tires. I said two will do on same axel.
The car seems to be tolerating this, but at times sitting in traffic, at a light, I get a kick in pants feel like the transmission is shifting hard into first. I think it is just the back axel and front axels unwinding kind of like a limited slip will do if something is not quite right.
It is almost time to replace the two oldest tires, so I’ll probably bite the bullet and get four new tires, as it is a pain in the neck constantly having two worn tires and two good ones practically during the winter in these parts.
I’m not a big fan of mixing tire sizes and brands on a street driven conveyance. The engineers designed the chassis with all four the same, and sometimes putting a taller tire on rear and or different makes will upset stability of vehicle. Different side-wall sizes will effect ride and handling as that flexing provided by side wall is part of the suspension’s design.
Track use only car; that’s a different story, it’s specialized, do what you want.
My 2 cents
Bill
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