The wife is going put some miles on the 80 this year. At 12 mpg and $2.00+ gas, I need better MPG's. If I go with P rated tires and light weight 265/80/16 or 275/70/16 (stock size, right?) should I get the 16 mpg range I had before the heavy LT285/80/16 lug tires? Does the 265/80 have the same diameter as the 275/70?