Ramjet 350 fuel mileage

I have a ramjet 350 i installed in a 69 chevy longbed pickup. i mated it to a 4l60e trannny and 4:10 rearend. It runs great but i am experiencing poor fuel mileage, about 7 mpg. the thing is i was getting about 30% better until recently. i have done top to bottom diagnosis and can find nothing wrong. i did a fuel injector balance test and the fuel pressure drop comparison between all injectors was within specs. but i was surprised the total average pressure drop was about 40 psi. does that sound like a lot, or is it just a function of how long the injectors run during the test? if anyone has any ideas, im stumped.

My guess is that its your rear end gear causing poor mileage. Also that series Chevy truck never did get very good mileage.

ok, but bear in mind i was getting better mileage with the same setup