[84LX89GT]

I've had two 5.0L mustangs and one 2.3L mustang here's what i had (all were 5 speed cars):
my '91 2.3L got 18-22 miles per gallon regularly (100k on engine)
my '84 5.0L LX got 16-19mpg regularly (150k)
my '89 GT gets regularly 17-21mpg (125k)
My GT also has 3.55 gears and a few mods that i would think reduces gas mileage (mileage is the same as when i bought it with only a few small mods).
The power differance easily justifies the purchase of a 5.0L over a 2.3L. I think the main reason i got the mileage i did was because a 2.3L has to work harder to move a 3000+ lb. vehicle so you have to floor it more which makes the engine pretty inefficient if it's trying to run richer for more power all the time. I used to have to wrap it out at full throttle and drop gears just to make it up hills, now i go part throttle in one gear and the torque just pulls it up hills.
i usually get 220-260 miles per tank of gas depending on when i fill up (hardly ever get to the "E" mark, but close) usually i fill up 10.5 to 12.5 gallons.