are your new tires outside diamater - larger than your stocks ?
if so then....
your computer is reading 1.3 ish miles as 1 mile.
think like this :
your stock tire = letts say 500 rotations to make a mile.
your new tire = 450 rotations to make a mile since its larger in diamater.
your computer of the car is reading how manny times the tire or drive shaft turns to equil a mile so its off now if you have switched sizes.
plus - for added stress - if you new wheel + tire size is larger than before - your final drive ratio at the wheels is off. so it is now much harder on your ****** and engine to go from stoped to what ever mph. you have effectivly added "highway gears" to your rear end
with the larger tire. this requires more power to start off from a stop, so there is your gas milage down the tubes. on the highway tho, it shouldnt matter much at all.