I’ve never paid attention to rear tire rollout but last weekend I measured. Rear tire rollout difference was 1/4 of an inch. Significant or not and what would be the maximum?
I’ve honestly never measured on my sprint kart, but it would depend on the track as to how much difference it will make. On a faster track I’d probably try to stretch the small one to fix it.
Since running MGs it’s less important, but there was sometimes an inch difference on the old Bridgestones. MGs are pretty consistent.
I always like to measure rear tires just in case. 1/4" isn’t too bad. But if it’s off, you’ll definitely feel it on-track. We always run the stagger so the bigger tire is on the opposite side of the most of the turns. Left-hand biased track = bigger tire on the right rear. Unless it’s a small difference, then I just send it. 1/4" isn’t that big.
I measure every set, especially if you have multiple sets for a race weekend, as you can generally match them within a few millimeters. 1/4 isn’t great, but it’s still within what I would consider usable. Generally most sets of MG’s are fine, but I did have one set this year where the rears were 5/8” different 
Ok so for the clueless… rear tire rollout?
Google:
Rollout is the circumference of the tire when it is is inflated to operating pressure. Matching the rollout of both tires on the axle means that each tire travels exactly the same distance for each rotation. Differences in rollout can cause the car to pull to one side or the other.Oct 8, 2001
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Interesting, never though about this before… KP ftw…
Question… how would one go about stretch the tire? My quess is… put it in the sun and inflate to a high psi pressure… inflate to race psi level and recheck?
That’s how I would do it, bead the tire and let it sit in the sun at the high pressure for a bit to stretch it. We used to do that with the Bridgestone a bit too if they were really wonky.
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Tried the high pressure but couldn’t get enough in to yield the tires to a larger diameter. Odd as it might seem, there was no sun in SoCal. It even rained.
A 1/4 of inch difference in roll out results in about .7% slip between the tires. Not sure what it means though but slip usually results in drag.
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