Do tires of different diameters, driven under identical conditions, wear out at the same number of miles?
There are a lot of other variables you need to take into account. Are these tires made by the same manufacturer? Do they have the same tread wear number? Do they have the same tread design? Do the “identical conditions” include the horsepower of the cars they are mounted on? Are they driven by the same leadfooted or featherfooted driver?
This is a very good question. Michelin X, the standard Michelin all season tire, has a wear rating of 140,000 km or 90,000 miles. That goes for the largest as well as the little 13" ones on my wife’s car.
Obviously the smaller tires rotate a whole lot more over that life, yet the wearout warranty is the same. I suspect they pick and average figure with a margin of safety in it.
In addition front tires will wear faster than rear tires. Perhaps if you clarify your question you will get clearer answers.
In theory, tires with larger diameters will last longer.
- BUT -
Most tire wear occurs in the cornering mode. Driving straight ahead is practically free. So when you look at tire wear, consider how many turns you make - and how severe they are - compared to how many miles you drive basically straight ahead.
The difference between a P205/75R15 and a P235/75R15 is only 6% - on a 40K tire, that’s only 2500 miles! That’s probably not enough to be able to detect.