If you changed form a six cylinder, and didn't change the rear end, the ratio is likely wrong.
the other thing is to check your exhaust system.
I had a 76 302 in an f100 and got 9mpg. Canged to dual exhaust, put on hi rise and 600 cfm carb and got 19mpg.
put a 360 into a 73 and could only get 10mpg with it driving at 60mph, but if I drove at 50, i got 17 go figure