You did not specify at what speed it is off by 4 mph. I assume 60 mph.
That equates to the speedo being off by .0666 percent.
Which means the mpg you are getting is only off by that amount. Which is nothing.
id say 14-15 mpg's sound aboot right with them tires you gots. i have the calmini 5" and 33x10.5 and get aboot 15 mpg's. once i throw on a front skid plate/ air deflector type deal, i hope itll go up a bit. at least make it a little more aerodynamic than a coke machine
Mathematically it's like a ratio. Oem tire is to aftermarket tire size the same as computer mpg is to true mpg. If you had 30" tires from the factory and now have 33" tires, you are making 10% LESS rotations going the same speed. That means you are going 10% faster than your speedo says and getting 10% better mileage than your computer say. So instead of your speedo being off 4mph, it's actually off by however much percentage more your bigger tires are than your oem tires.
If you were getting 250miles in a 15.00gal fillup in the stock tires, now with the taller tires with 6% difference, you actually traveled 266miles.
stock tire/speedo. that's 16.67mpg.
corrected distance with taller tires, 17.77mpg
this is not accounting the heavier weight of the tire, so you'll prolly hit just 240miles or less in that 15gals of gas.
which is about 0-.5mpg difference.