Adjusting MPG's for Tire Size

Just curious, but I thought I read somewhere on here that when increasing your tire size, you need to add a certain percentage to your new MPG to adjust for the size increase. In my mind, this makes no sense as the speedometer and odometer gear / calibration did not change. So as far as the truck is concerned, the rear end still only turns a certain number of times despite how far the truck goes. Basically, I'm trying to figure out how many miles to the tank I lost when I upgraded from the 245/75r16's (stock Dunlops) to the 265/75r16's (Hankook Dynapro's). I was averaging between 390 and 400 miles to the tank before and now I'm getting around 370-380 per tank. Should I be increasing the new miles per tank to be accurate?

Just for the record, I understand this is a truck and gas mileage will be low. I'm actually not concerned with it especially being I only commute 7 miles each way to work and my old '99 Taco only got 16 mpgs. This is more for OCD (or CDO if I arrange the letters in order like they should be.lol).

Tried doing a search for it but came up with nothing.

 

Using the tire size calculator, you'd have to add 3.7% to your ODO. (Use your ODO x 1.037 = Actual mileage)