Before I begin, I know ... I know. "351M" and "fuel mileage" don't belong in the same sentence. However, I'm a glutten for punishment -- so here goes...

What I have is ::

1978 F150 XLT Ranger Regular Cab/Short Bed 351M/C6 4x4 (Axle Code H2J)

As far as the drivetrain, it is bone stock. All of the original vacuum lines, EGR valve, air cleaner, etc. is in place. It's all original, never rebuilt -- only maintained.

Now, since I've semi-sorta put the truck back on the road, I've been averaging about 8mpg with really bad 265/75/15's, a worn accelerator pump, and warped rear brake drums.

I have now put on new 235/75/15 street radials, rebuilt the accelerator pump, installed new brake drums, and have yet to check the mileage. Also, I have installed a new power valve in the carb, changed the fuel filters, checked the plugs and wires, and also checked for vacuum leaks and found none.

When I verify my mileage this next trip, I'm going to keep idling to a minimum and use my GPS to track the miles.

To get down to business, I'm wanting to see what everyone's 351M engine is getting, especially stock units. I know there are several guys on here who have bought their trucks new, or had them way back when, and can remember what they used to get.

I'm looking toward the future, trying to decide if I want to milk every mile I can get out of this 351M, build a mild 400, or swap in a 12-valve Cummins. If I go Cummins, the truck will be swapped to a 5 or 6 speed manual. I'm in-between swapping in a 4-speed with the 351M or the 400. When I think on this topic, I try my best to keep cost-effectiveness in mind, and with that being said, the milking the 351M option is the most cost effective. However, if it'll get 10mpg at best, and a 12-valve Cummins will get 20+, it still doesn't rack up as quick with the major difference between the cost of gas and diesel fuels.

My Granddad, who bought this truck brand new, said in it's hey-day, it got 13mpg. I would really like to see that again, but my hopes are not set above 10mpg...

I did several Google searches and some searches here on the 351M fuel mileage topic, and I couldn't really find anything specific like I'm looking for, so here goes...

Any thoughts?

 

I have a truck similar to yours. I can tell you now that you may at most get 12 mpg with it being stock. I recommend that you keep the 351M & put a ZF5 (there is a swap thread for this in the 1980-1986 truck forum) behind it if you're looking for better mpg. You might want to rebuild the carb or check the fuel pump if it has been sitting awhile.

 

Best I've gotten in mine is 12.6, purely highway driving. Heavy city traffic, short drives, I'll get 6-7.

 

All great advice, thanks!

I had forgotten all about a ZF5! They ran those behind some 460's. Only thing I was dreading was driveshaft work.

I forgot to mention I replaced the fuel pump 1st thing.

 

12ish seems to be a pattern.

At what speed, with a C6, have most of you found that your mpg starts to go South? I'm thinking above 55-60mph.

 

You've got it right from what I hear. It is best to keep the speed below 55 mph (otherwise the noise from the engine revving high [around 3k rpm] will annoy you) . I personally can't tell if it is true because my truck ain't street legal yet (But, I did drive it twice).

 

I know how you feel on the street legal. It took me a little while to get mine ready to tag and insure, but I had to sneak those quick rides every now and then to pacify my urge.

I've pushed 70-75 a few times and she's screaming. I know it's really sucking down at that point.

 

That depends entirely on the gear ratio in the differential. That 12.6 was in hilly Virginia terrain, speed limit varying from 55 to 65 and driving about 5 over. I also have my truck geared 3.50:1, with stock tires.

 

I'm wondering what gear ratio I have. I've got the axle code in my 1st post if anyone can decipher it.

 

Ford 9" limited-slip geared 3.50:1.

Decoding '73-'79 Ford Truck VIN Tags - FORDification.net

 

I only had 400s and a well tuned nice running 400 would be in the 13-14 range with 350-354 gears and a 4 spd. I did buy a 79 f250 with a 429 cobra jet in it from a guy that had a drilling company and he probably logged more miles in dent sides than anyone I ever found. The wild cat drilling fleet was comprised of like 12 dentsides at one point all powered by 351, 400, and 460s and all 4x4. He said at one time he actually tuned a 351m in to 17 mpg with dual exhaust, hwy miles under 60 mph, and a 4 spd with 354 gears and running empty. I had no reason to doubt him. I have made 12 with strong 460s before on gps calculations before, but you better be ready to tune and be light footed. I would say yours might make 12, if you do cap, rotor, plugs, wires, tune the carb, mke sure your exhaust is good, play with timing, put a good fan clutch in, do a good thermostat change, lean down the carb if possible, new air and fuel filters, make sure your advance in the distributor is working right, maybe even do a tranny tun e up, new fluid and new modulator, remove valve body and clean it.

Anything that can rob power needs to be addressed for fuel mileage. A new timing set might help, if yours is old and stretched. My c6s are awesome units, but they have had governor issues as well as modulator issues, if your tranny is shifting wrong, or not at all, you are starting in high, or not coming out of low soon enough, it allkills mpg. Good luck.

 

12-13 ... maybe 14 .... on a good day east bound from Dever, Co. to Anywhere, Ks. with their gearing, brick like aerodynamics, and no OD.


I once did squeeze mine (4X4/351m/C-6/3.50/31" rubber) to right at 16 on the open road at 55-60 MAX. I was playing with timing and the jetting and power valve combos in the Holly 1850 600 cfm carb ....
.... I was rewarded with some burnt valves after enduring some period of sluggish throttle response. It was in 1990 just near the end of summer as I started building my house and I had to pause to fix my truck's heads.

But it was a valuable leaning experience. I'ld love to stumble up on a nice OD unit someday just for grins, maybe if I won a lottery?

 

Thanks a bunch!

 

Thanks for your response!

Back in '03, the plugs, wires, cap, and rotor was replaced. This 'ol dude hasn't seen over 200 miles since then. I pulled the plugs a few months back to make sure they weren't fouled from a failed power valve. They still looked new.

I haven't gotten around to checking the vacuum advance yet. I found an article on that not too long ago . . . I'll try and dig it back up.

Now, the tranny, I know nothing about. I'm going to do some searching for good articles on carb tuning and good 'ol basics of a Ford C6.

I don't drive this truck very much. I have a 53 mile trip to work and back, so I have a little commuter that averages about 37mpg for that. It would be nice to drive this truck to work a day a week, but 8-10mpg just doesn't make it feasible. If I could squeeze 13-14, not that it's very much better, it would be more doable.

I have garbage pick-up here in the city, and I don't have any house projects going on after a face lift two years back. I sold my tractor and all of my equipment when we bought this house, so I'm not towing it here and there anymore. I really hate to say this, and never thought that I would, but having a truck is purposeless in my situation, at this point any way. However, this was my Grandfather's, and it's not going anywhere. Now, since I've gotten it mechanically sound, I'm going to start driving it 10-15 miles a week to keep everything turning.

I started this thread because I was curious what other "stock" guys are getting, and to see what I could possibly "tweak" for better mpg once I verify that the miles driven are recorded correctly.

 

Thanks for your input!

Too lean is what scares me when it comes to fine-tuning a carb. Fixing to make a point to search for some good carb articles here in a minute...