18 mpg in the 77' long bed 2wd AT
15 mpg in the 78' short bed 2wd AT
10 mpg in the 73' short bed 2wd AT.

Used to have a 79' F150 4x4 short bed with a cammed 429 CJ, AT and 38.5 Ground Hawgs... Better measured in GPM... Gallons per mile!

 

OK Swapping to the gear ratio you mentioned will bring you 70 mph rpm to 2366. Any time you can lower the rpm without going to low you will be burning less fuel but there is a trade off. You will loose a lot of off the line acceleration. The truck will feel heavier as you drive it, Truly the best thing is a Overdrive transmission, there are adapter plates made for the FE motor to accept a modern 4spd automatic overdrive transmission with electronic locking torque Converter. It will be more expensive in the long run but you will retain you around town driving you have grown accustomed to while lowering your highway RPM.

I suggest a E4oD transmission out of a 92 to 96 f150 this is the 4spd equivalent of your C6. It is electronically controlled and there are stand alone controllers available at a reasonable price.

Your gear ratio at 70 mph with a E4oD swap with 3.50 gear is 2024. depending on where you live this would be a little low for my liking cause I live in mountainous region. I would deepen the gears to 3.73 or 3.92 and still be between 2157 and 2267 rpm at 70 mph.

Another thing is aerodynamics, there is not that can be done about this other then lowering and some sort of bed cover like a shell or tonneau would help slightly.

 

Thanks for the info Grimm13... I have only driven mine about 300 miles so far so I am not accustomed to much, ha ha. Right now I have sandbags so it doesn't spin out so much on the gravel, so a little less go on the low end would be OK.

And I may be able to get the gears swapped for around $300 to $450, where the e4od and controller would be much higher investment.

Thanks again, I will be thinking and researching...

 

the gearing change may not be the saving grace your initial thinking is, your torque converter will load up more before the truck starts to move and the line between slipping and not slipping may become very fine.

 

In researching I have also read that the FE engines like to rev a bit and that 3000 rpm doesn't really hurt them... and I remember reading that if your cruising rpm is close to stall speed of converter that you will end up with too much slippage too.

Most of my trips will be 45 mph on 8 miles of gravel and 55 mph on 6 miles of pavement to town... the "big" city is a other 25 miles with a 65 mph limit. Probably only get there once a month with the pickup...

Might be best to stay with what the factory did!

 

That is why I like electronic transmissions (I may be even more partial being a Electrical Engineering student) but I believe that most once in second gear go into a converter lock up which eliminates most of that parasitic heat build up you mentioned from torque converter slippage.

 

If I were going with an OD transmission, I would prefer a 5 or 6 speed manual. Again, cost prohibitive at this point for a pickup I paid $350 for... but fun to dream!

 

apparently the 1978 to 1979 frame motor mounts for a 351m & 400 will work with a 351W, so you may be able to get a mid 90's F150 351W with a ZF5 and get fuel injection at the same time.

 

I was thinking a 2.3 Ecoboost and six speed out of the new Mustang... or swap in a 4bt Cummins to improve mileage... as long as I am dreaming!

 

The 4BT with a 5R100 transmission out of a 6.0l ford superduty, a NP271 manual transfer case and 3.92 gears is the set up I am working towards in mine. The ratio will be 1859 rpm at 65 mph with a 33" tall tire which is just about perfect, in the torque band with enough to get you higher if you choose. Personally I go a little slower, its a little quieter and I plan ahead.

 

Steven@nd Fe's work great with 2.75 gears i avg'd 16 mpg for an entire year of daily driving with a healthy Fe (pnp heads and aluminum intake, headers, and 4bbl 650 holley), 435np, 265-75-16's and she wasn't a slouch i hauled firewood all the time with it essentially 3rd becomes 4th and 4th becomes overdrive...

 

8-10mph 351W 4 speed, 4.10 gears w 37's. I'm thinking a 460 could match that.