15" wheels can fit up to 31/10.5R15 which is nearly the same as 255/75R16 for 16" wheels.
Some people say that big won't wear evenly. Some say they do. They will cause your speedometer to read a few MPH lower than actual speed, which also falsely calculates out to lower fuel economy, but they will also really achieve a little lower fuel economy.
A little more conservative might be 30/9.5R15 or 255/70R16.