I thought mixing tire brands or sizes on the same axle was bad enough. Now comes someone that wants to put 16" wheels on the back and 15" wheels on the back with probable mixed tire brands and sizes.

What a bad idea! You know when you are driving down the road you owe it to the other people on the road to drive a safe vehicle. You should have 4 wheels that are the same size in diameter and width. Then you should have 4 tires that are the same brand, size, and model no. Anything else is un-safe, unless you have a car with a staggered wheel width configuration.

Try to think of this mental image. Everyone has seen it. The small whizzing down the interstate at 75 mph with one of those miniature spare tires that is limited to 55 mph. You will sort of that dangerous with your proposal.

Steve