So he has roughly a 28" tall tire based on what I see. A 225 is just slightly over 28" tall.
As for MPG calculations...it is as simple as fill up the truck, note the mileage, drive it through at least half the tank full, fill it again and note the mileage again. Subtract your first fill up mileage from the second and divide that by the gallons used.
200 miles traveled - divided by - 10 gallons = 20 mpg