It depends some on how you drive it and what it's used for. What transmission and rear gear ratio. I get between 12 to 14 with my 73 F-100 , 2 wheel drive. It's got the 302 , 2bl carb ,3 speed transmission and rear gears are 3:25. You having a F -150 , a lot of them had 3:50 rear gears if they a 2 wheel drive. You should get about the same as i get i believe. If it's a 4x4 then i don't know. I should be getting better than what i do.
If I put 3.00 gears in the rear what gears should I put in the front to balance it out my f150 is 4wd with 6 inch lift and 36's
You need to put the same ratio in front and back. 3.00 gears with 36 inch tires and a 302? Honestly, with that combo your truck wont be able to get out of its own way. If you want your mpg to come up, you need to dump the rotating mass of those tires. Maybe 32/33 inch tires, but with a 6 inch lift you might not like the stance of the truck. Another thing to keep in mind theres no aerodynamic qualities on our rigs. They are what they are. My combo is 351M with 33 tires and 353 gears. I get 8 around town, and 12 on the highway if I keep it about 60
That might have been good to mention in the original description of your truck... No wonder your getting poor mileage, your running 36s. Your driving a lifted truck from the 70s with over size tires, your going to get terrible mileage no matter what you do. With 36s you might be better off going with a lower gear ratio so the engine doesn't have to work so hard. 4:56s would probably be your best bet, you could probably get by with 4:10s. I don't know what your ratio is right now.