My 77 F150 4x4 with 351M and granny four speed gets about 8 MPG. I know something is wrong, but don't where to start looking. I recently replaced the headers and dual exhaust, cap and rotor, and spark plugs and rebuilt the stock carb so it would idle. Next will be the ignition box. I have the timing set by ear, and sometimes the motor won't turn off when I shut off the key. I want to tune the carb and egnition, but need the light and vacume gauge. My question is this: I can spin my 33's in first and second gear without even trying. I am wondering if the ratio is super high, or is that normal with a granny 4sp? Since the rear end is leaking(the seal down by the banjo), and dad told me I had to pull the axles anyway, should I put in lower gears and a limited slip? How much would something like that cost and where would I get it? I live in hawaii and parts are super expensive, with no junkyards carrying my truck (I checked them all). I know how to figure out the ratio from previous posts of BBT (thanks), but I haven't jacked the rear end up to figure it out yet. I love the truck, but at $40 every two days, it's killing me! Please help as I don't want to go back to my nissan!

 

Well, 8 MPG from a 351M on 33s is pretty normal, but spinning those 33s isnt. Ms usually dont have the power to do that. If the engine "runs on" after shutting it off, you probably have the timing advanced too far (this may explain why you can roast the tires. Setting the timing to around 8 deg initial advance may give some better MPG, but not much.

I would think that for the cost of changing gears you could buy a lot of fuel, considering what little difference it would make.IMHO

Nathan

 

You want to install "higher" gears not lower ones. Lower gears would just compound your problem.

higher gears are lower numerically.

If higher gears weren't worth the effort then why have overdrive transmissions?

I believe a gear swap would easily put you over 10 mpg. And it does add up over time.

Even larger tires have the same effect as well, but do add weight.

Josh

 

Well, I get between 9 and 11 with a 93 302. So I'd say you are right where you should be on the mpg thing. 'Course my 78 F-250 with an 300-L6 and 4.10:1 gets between 17-19. Both 4x4's.
Garett

http://members.tripod.com/mephisto_xes

 

Garrett, how are you gettin' 17-19 with your six? I've got 2.73's and my truck is gurgling it down lately. Know any tricks?

 

i disagree with 8 being normal....my 77 with a 400, granny, and 33s gets 11-12....that's with 3.50 gears....the correct gears for the 33's would be 3.73s, but it sounds like he has 4.10 gears...along with a need of a tune....when i had 31s on the truck, it got 10-11....

 

I agree, with the old 360 in my '71 with 4.10s and the classic NP 4 speed in there i was getting about 8-10 MPG running on only 6 cylenders, how? wellll i just keept my FOOT! outa the gas actuator. Roasting tires, well thats easy actually with the 4.10s and a 4 speed. POP-clutch and stand on it in 2nd and if you can keep your rear tires from hopping then sure you'll roast em (33 mud tire lugs make roasting hard the lugs catch and release making you get wheel hop) and if you have a big-block with that damn monsterious tourqe then you'll proably fry clutch too, i did that on dirt once.. had to wait 5 min to get it outa gear.. ahh the smelll.. the smell.

 

as a foot-note i'd like to add that ford waaaayyyy under built the clutch in the older FE's with NP trannies, the transmition is almost perfect at getting all that tourque to the ground but you have a pretty small clutch for that much tourque and the bigger 390 had only 1/2" bigger clutch disk! not much considering the extra tourque that added to the system. Even with normal driving my last clutch lasted about a year and that is un-burnt ( i.e. even wearing on both sides of the friction disk) then again i spent most of the summer hauling 1-2 ton loads of DnG arround in my bed. I love old fords, only 3/4 ton i have ever found to be able to be that far overloaded regularly and only blow the shocks. The tonnage numbers are real considering Granit Rock weighs the truck in and out to get the price of the load.

 

Ok, Ok, Ok already. I guess the 8 mpg sounds normal. I still want to try for higher milage, and I guess I am partial to a limited slip rear-end. I haven't been off-road with the truck yet, and I am sure the granny works awesome, but I still want to screw with the rear end. (since I have to replace the seal anyway). Thanks for the opinions though, I enjoy hearing from you other motorheads. Mahalo.
p.s. Anyone have a favorite limited slip?

 

http://www.4wheelparts.com/ This should get you anything you need. You can shop on line or order a catalog. Auburn is a good limited slip if that is what you are looking for. With 33's 3.73 to 4.10 gears is about right. On my 77 F150 4x4 C-6 with 351M, I run 3.55's with 35's I dont have much off the start but have a great top end once you get her rolling. Still get about 10 mpg...up hill, down hill, with the wind, against the wind, putting my foot in it or being nice.