Morning All
Okay - I got a little geeky this morning after reading the above comments and decided to do some math. Here is what I learned: 315's
may negativly change MPG, but not by much, if at all.
Looking at Bridgestones specs the Dueler AT 285/75R16 turns 633 revs/mile. Per BFG the 315/75R16 AT KO turns 601 revs/mile. This gives a "mileage factor" (MF) of 1.053244592. So for every mile the odomiter indicates, multiply that by the MF to get your actual mileage traveled.
You can see in the screen shot below what the adjusted mileage numbers are with the corresponding MPG. It really doesn't seem to make much of a difference.