This is the 1st post I have done here & only a couple before this on a firearms site, but here goes, what the problem is bad MPG, I got this truck to give my grandson when he starts drivin, have done a lot of reading here and think i'm stuck with what I got. Truck is a 1987, f150, 351w, 4bbl,3.08 gears, c6 tranny. RPM 2600 @ 65, 2400 @60, actually went down from 2800 @ 65 when I put these tires on it, tires now are 235/75r15 front measure 27 1/2 " tall, rear 31x10.50 LT measure 29 1/2", speedometer is 5 mph off now (slower ) I have taken off all pollution stuff, it has new carb, radiator, plugs, wires, cap, rotor. Can anyone think of what I might do to increase MPG, only about 8. any ideas would be great Thanks in advance

 

Swap in an overdrive transmission and then switch to 3.55 gears. That should get the fuel economy at least up to 12 if not better. Also, if the new carb is too large that wont help either.

Since the speedometer is off a little, the odometer is off a bit too. Its probably getting a little better than your figures show.

 

You say you removed all the pollution stuff, does that include the stock cat and heat riser diverter valve? If not get those things off ASP and replace with an aftermarket cat or resonator and remove the butterfly valve from the diverter. The old pellet cats that were installed on these trucks were prone to melting into a solid mass and plugging the exhaust and I'm not sure what logic was behind putting a stupid butterfly valve in the exhaust flow but if it ends up seizing in the closed position the motor will be badly restricted. The factory 4bbl motor in this vintage truck was considered a HO and it will produce respectable power and milage... if it can breath.

 

Thanks for the replies, I actually started reading about putting a od tranny
last nite, which 1 be the best one, least labor intensive & price. The new carb is a holley 80457-7 &if I looked it up right it is a 600 cfm, would that be about the right size ? I didn't even think about the cat converter, that will be gone SOON. I know I wont ever get good gas mileage, but this is worse than the 77 f250 with 460 & c6 that I had, thanks for the help
can yall help me understand this truck with 3.08 gears, having so high rpm's, or at least they seem high to me, I am learning a lot on this forum, never been a mechanic, but like tinkering with these old trucks, I can usually em running, but don't have a clue about What works best with what, just trying to rig up a good sound truck for my grandson, one he can afford to
drive THANKS AGAIN