Okay Coastie, thanks for the full-disclosure update! You are certainly calculating your mileage correctly. But your extrapolation of how many more miles you could have run if you ran to empty that gets you over the 400 mark is what was so unbelievable to us. I don't think it's legit to thrown that in, because the 22.5 gallon tank rating does not translate into 22.5 gal of usable fuel... I can put in 21 gallons if I am sputtering and stalling pulling up to the pump. I am pretty sure any of us will run out of gas before we can burn 22.5 gallons.

Here is a tire-size-discrepancy calculator. If you calculate the difference in circumference on your bigger tires and multiply your odo's mileage reading by that ratio, you should end up with a more accurate mileage calculation.