Larger tires = more turns of the new larger tire to equal 1 revolution like it was with stock tire size.
Say for example I did 230 miles on the trip meter:
That's 230 x 1.12 = 257.6 actual miles
Divide by gallons of gas gets you your MPG's
Your odometer reads slower because of the larger tire size. Same with the speedometer, its off by a percentage, so remember that its just not 5mph slow because there is a big difference between 12% of 30, and 12% of 70.
If you wanted to be really accurate you could measure the actual diameter of your tires vs the stock size for an exact percentage.