DON'T DO IT! 
I purchased a used AWD M3 last November, and the prior owner apparently didn't understand the importance of rotating tires, so the rears were nearly bald when I got the car. But the front still had most of the tread.
I finally decided the rears were no longer safe, so I replaced them with 245/40/18 Michelin Pilot Sport AS3+ tires (and moving the original 235/45/18's to the rear). The diameter difference between these two is .7" and the rev/mile difference is 789 vs 766. I wanted the wider tire for rim protection (which it really does cover much better) and for a slightly lower stance for highway efficiency. My plan was to burn through the old tires on the rear quickly then replace them with the new tire size so they all matched.
I had considered that the mixed sizes might cause the Tesla some issues, but thought the very small difference wouldn't cause a problem. I've mixed tire sizes on other modern cars (BMW 3 series) with traction control before without trouble. BOY WAS I WRONG! Above 35-40mph, the traction control light flashes constantly, and above 50mph, the car randomly jerks like it's breaking - I'm guessing the traction control is trying to "correct" the "slipping" wheels...whichever it thinks are at fault. Needless to say I didn't drive the car long in this condition - I immediately returned and had all the same size installed.
This car was so sensitive to this, I would even be concerned about spares...if they are not exactly the rolling diameter of all the other wheels on the car, I would suspect you would have problems - maybe even dangerous ones.
I purchased a used AWD M3 last November, and the prior owner apparently didn't understand the importance of rotating tires, so the rears were nearly bald when I got the car. But the front still had most of the tread.
I finally decided the rears were no longer safe, so I replaced them with 245/40/18 Michelin Pilot Sport AS3+ tires (and moving the original 235/45/18's to the rear). The diameter difference between these two is .7" and the rev/mile difference is 789 vs 766. I wanted the wider tire for rim protection (which it really does cover much better) and for a slightly lower stance for highway efficiency. My plan was to burn through the old tires on the rear quickly then replace them with the new tire size so they all matched.
I had considered that the mixed sizes might cause the Tesla some issues, but thought the very small difference wouldn't cause a problem. I've mixed tire sizes on other modern cars (BMW 3 series) with traction control before without trouble. BOY WAS I WRONG! Above 35-40mph, the traction control light flashes constantly, and above 50mph, the car randomly jerks like it's breaking - I'm guessing the traction control is trying to "correct" the "slipping" wheels...whichever it thinks are at fault. Needless to say I didn't drive the car long in this condition - I immediately returned and had all the same size installed.
This car was so sensitive to this, I would even be concerned about spares...if they are not exactly the rolling diameter of all the other wheels on the car, I would suspect you would have problems - maybe even dangerous ones.
