keep in mind that if the tires are larger than stock and you don't correct the speedo you will get a false MPG reading.

here's my example.
my tires are 24% larger than stock (215/75/15 up to 35/12.50/15)
my gear ratio is 30% deeper - i had to re gear from 3.73's to 4.88's to turn the tires.
that is a net difference of 6% to the engine - this compensates for the larger circumferance (also known as roll out) and the greater rotating mass.

when my speedo reads 50 mph i am actually running at 47 mph. not much of a difference, but it adds up.
off a tank of gas if it shows 300 miles for the tank i actually only got 282. which changes my mpg average from an indicated 20 mpg to 18.8 mpg.

in your case, if you put larger tires and didn't change gears or recalibrate the speedo, then you will show a slower speedo reading and less miles per tank.

not knowing your drivetrain specs this is just an arbitrary example.

tire size change becomes a 10% net difference
at 60 mph indicated you are actually traveling 66 mph.
at 300 miles for a fill up you calculate 15 gallons and come up with 20 miles per gallon (yeah i know it's a dream)
you actually went 330 miles which calculates to 22 mpg.

when i was hot-rodding dodge trucks with the cummins diesel engines there was a list on the forum that had real world numbers showing the MPG loss by upgrade
1mpg per 1" of lift
2mpg for 4wd vs. 2wd
etc

changing the load, tire size & weight of the vehicle will certainly change the fuel economy but you also have to factor in driving habits (quick acceleration), speed, terrain (lots of hills?), weather (vehicles always get better fuel economy at sea level in cooler temps) & time of year (the gas stations sell different mixes of fuel in different seasons.)