I have a 78 F250 with a 400ci, C6, and a Dana 60 with 4.11 gears. I've read many related post on here about trying to improve fuel mileage, and most solutions have a pretty steep price for pretty small results.

1. Carb to EFI - Options for this have improved over the years, but it's still at least $1000. These things still seem to be far from plug-and-play, and don't really gain much.

2. Transmission swap - This an expensive option, and you lose some system strength with an AOD or E4OD.

3. Gear vendors OD - Expensive.


So, that leaves me looking at that massive rear end. I know there are a lot of guys out there who are trying to go more in the heavy duty direction and would want a Dana 60, but I am trying to move toward a more efficient daily driver, and I don't have much need to tow anymore.

Is there anything lighter out there that would be a clean swap with about 3.50 gears and 8 lugs? I suspect there isn't. I am guessing a 5 lug 9" would be the closest thing two a clean swap.

 

Nothing you list would ever be cost effective.

Consider, next time your DUE for fluid changes, use synthetic, if acceptable(diff, trans).

When due for tires, install highway tire, check rolling resistance ratings when shopping tires, at higher PSI (35-40 if rated for such).

Have the timing /curve possibly reset for a little more advance and keep it tuned properly.

Drive w/ moderate acceleration and plan braking as much as possible.

Buy your fuel at Costco, or Arco, for lowest price.

Don't expect much.

 

You can get some significant improvement with a wideband 02 sensor and tune the carburetor. It won't be EFI but should be able to pick up 3 or 4 mpg, depending how buggered the tune is now. It won't run any different and most likely, a lot better. It's amazing how much fuel is wasted even when it "runs good". My old Y block went from 9 mpg to 12 mpg after replacing the Holley 500 CFM with an Autolite and then from 12 mpg to 16 mpg using a wideband for jetting. Might be tough for a 250 and 4.10 gears, but this is worth doing. Careful distributor curving of the ignition advance will pick up a couple mpg as well. This costs next to nothing other than your time, and it will likely run a lot better.

 

1. The EFI swap is more for driveability than MPG.
2. A manual trans is more efficient than an automatic. With 4.11 gears, a Tremec TKO-500 with a 0.68 overdrive would be ideal.
3. A rear axle or rear gear swap would the most cost effective... 3.54 is apparently the lowest gear available. https://www.summitracing.com/search/...tOrder=Default

I do not recommend swapping the rear axle for a 9-inch due to the lug pattern mismatch.... then you'd have to carry two spares (added weight).

Learn to "hypermile": 100+ Hypermiling / ecodriving tips & tactics for better mpg - EcoModder.com

Have you considered buying a more efficient daily driver like an early 2000s Ranger?

 

When you factor $$ spent VS. actual MPG gains, only simple things pencil out. And don't do much at that.

 

I supposed I'm not familiar with using an O2 sensor on a carburetor engine. How does that work?

I actually installed one on my truck a few years back to use with an air/fuel ratio meter, but that's it. Is that what you are referring to? It was a cheap version, and it was only able to use it to point me in the right direction.

 

didn't someone mention somewhere that the compression ratio on the 351m and 400 hurt economy?

 

Sounds like it might have been a narrow band. Widebands are a lot more useful. Plugs are tough to read without a lot of experience, and even then it's still a bit of a guess. A wideband takes all the guesswork out.

 

I had a 76 f250 Camper Special with 460ci and C6 with street/strip shift kit.
Engine was stock except for Carter 625 carb and an after market intake (not sure of intake name, something like mileage master) and gear ratio was something like 3:07

Empty or full, the mpg didn't change much.

Loaded with an old, heavy, 11.5' camper, pulling a car trailer, 4 seat sand rail, spare parts, full rollaway and lots of race fuel (2500 lbs overweight on rear axle) I got mpg in the 12s, better than my 18,000 mile 88 f250 xlt lariat, efi 7.5, c6 gets empty.

 

A friend worked at a dealership in service dept. way back when they started making the mfg. put the epa ratings on the vehicles. Ford had a list that would tell how much all the bolt on pieces effected the mileage. Big mirrors, running boards, bug deflectors, bed covers all subtracted about 1 mpg each my friend said. Just tire pressure seems to make a difference in my work beaters.

 

On a budget: good maintenance, moderate tires (not too wide and no big lug tread pattern), plenty of tire pressure, avoid high speeds and drive it as much as you can afford. A front air dam under the bumper may also help.

There are lots of good reasons to make significant upgrades like fuel injection, rebuild for higher compression, overdrive ect. Performance, altitude flexibility, noise, fuel flexibility, drivability and range for example.

Major upgrades can also help fuel economy but "return on investment" for just the fuel economy benefit is kinda limited I think.

 

I've got those massive west coast towing mirrors that I plan to swap out for the standard ones, mainly for cosmetic reasons, but that should be worth a good 2mpg alone in lessened wind resistance.

 

1) Wideband O2 sensor (I like 'Innovate' sensor gauges)
2) Good electronic ignition and .060" gapped plugs.
3) Headers for improved exhaust scavenging.
4) Good dual plane intake for well mixed charge.

Set your carb up so cruise is between 14.7 and 15.1 to 1 and it'll give good cruising economy.

Now shift your mixture on acceleration to be around 12.5 to 1 or so. Under that is rich, over that can tend toward lean. Anything over 13 bring it back down immediately.

Holley 'Street Avenger' carburettors are tuned nicely out of the box and gave me a good improvement over the stock carburettor on my old Chrysler. EFI is too expensive for my tastes but does bring tuning advantages.

Cheers - boingk

 

At cruise up to around 16 to 1 or so (carbureted engine) is probably doable. Just check for signs of surge or misfire. The mixtures won't be equal in every cylinder. That was the old school way to jet - lean it out till it surges at cruise, then jet back up two sizes. Going too lean will actually reduce fuel economy. I was able to drop five jet sizes from stock, and that was in cooler weather. Holley jets only go down to #40, running #43, each jet size is said to flow about a 2.5% difference. Judging from the bump in economy I saw, this is about right.

This is the important part. Can't ever go lean on acceleration or under load. Will break something or burn a hole in a piston. Cruising on level ground, very lean mixtures are OK, so long as it doesn't run too hot or misfire etc. Leaning out the cruise jets though, also causes a leaner wide open throttle mixture. Consequently, have to hog out the power valve channel restrictors slightly with a drill to get the wide open throttle mixture back into the safe zone. This works slick as hell though. Lean cruise mixtures won't work, I'm told, if you run a catalyst, it will burn up. But this was the basic method for getting good economy before emissions mandates made the manufacturers detune everything in the smog era. I guess the general idea was to allow more HC (unburned fuel) in exchange to reduce NOX (smog). Stoich is theoretically perfect, but best lean cruise spikes NOX. See chart.

 

I'll just add to the list front end alignment... especially toe.

Scrubbing front tires will pull down fuel economy.