I am only getting 6 mpg on my 1980 F-150 with a 400 cu. in. It has never run so smoothly and quietly but just gets terrible mileage. Was getting about 12 -14 on gas. Any ideas?

id liek to know how you got that mileage before, my stock 6.6L 400 only gets 5-6mpg with 350k on it

my 400 was pulling 10 mpg, with 39.5's, 1tons and 5.13's no overdrive in the tranny @ 70 mph, 3000 rpms.

Details of the propane system?

my 400 was pulling 10 mpg, with 39.5's, 1tons and 5.13's no overdrive in the tranny @ 70 mph, 3000 rpms.


i was pulling 5-6 with 3.55 gearing and 33's, stock np435 and 205, dunno what i get with the new 4:10 and 38's yet

Hmm.. my 400 got 9mpg on gas an gets 9 now on propane. Have you gone through the timing thing? The timing is definitely different for propane. To really maximize it you're supposed to change some springs in your distributor... I haven't done that yet though, I just messed with the timing till it felt right. There's info some place on the net if you search..

Oh, I have 3.55 with np435 and 31s.. all about to change tho :)