Mismatched Tire Question

[NH_RPS13Drifter]

Ok, everyone who knows about drifting knows that it's an expensive sport. One of the major factors in this is the costs of tires. Luckily I have found a garage that gladly gives me all their old tires because they would have to pay to get rid of them anyway. As a result of this, I get many different tire sizes. I have picked up everything from 205/65-15s to 185/65-15s, including all the other 15" sizes in between.

My question is this:A few days ago I was running two 205/60s in the rear of my S13 when I put too much lateral force on the right-hand rear tire, causing it to blow out. I was low on tires at this time and had no choice but to on put a 205/65.

I know that if you run tires with a bigger diameter than stock, you will auctually be going a bit faster than the spedo says, and if you run a smaller diameter you'll be going a bit slower... But what if you have different sizes?

For example: My left tire's diameter is approx. 20.5mm or 2.05cm bigger in diamiter than the right tire.

This is what really confuses me. Would I just use the larger tire to estimate the speed deviation of my car, or would I average the sizes tograther, or what?

btw, I have an open diff which annoying, but kinda good in this situation because I dont have to worry about getting wheel hop a higher speeds while running different tire sizes.

Thanks a ton!!

[chmercer]

you would use the smaller tire to figure out your spedometer iirc.

[NH_RPS13Drifter]

How is that? I would think that it would be either the larger tire or the average size between them. I was under the impression that since the diff is open, both tires would spin at the same linear speed, there by forcing the smaller to increase it's rotational velocity in order to match the rotational velocity of the larger one, in effect making the larger tire the one to go by. Jeez, I don't know. Can anyone help explain this to me?

[Exar-Kun]

You are correct. THe smaller tire will be forced to rotate quicker, almost like you are taking a turn.

Luckily you do not have an LSD..you'd be messing it up something feirce like that. get tires of the same dimensions soon please..right now your car is rolling like a solo cup on its side....

-Chet