What would work better as an equivalent size for my spare? I have the BFG 275/65/18's and a 17" spare wheel.

275/70/17 or 255/75/17

While the 275/70/17 is a better width match, the 255/75/17 is actually a better diameter match. Not by much though.

Obviously my spare won't be getting a whole lot of use, but it would save me some time and effort if I get a flat. Thanks.

 

The closest diameter is going to be best. It should be as close as possible if on the rear so as not to mess up the gears.

 

Definitely the 255. There is only 1/100" difference with the 275.