1976 F-150, 390, C-6, 3.5 gears, 235's

I was if anyone could tell me about what my highway mileage should be with this truck. I get 7-8 in town, but the engine spins pretty fast at 55.

 

I think you need a tune. I get 8 - 11 mpg on the highway with 3.70 gears. Engine is no where near stock. 3200 stall converter too. In town is a different story and quite a joke.

 

I have heard claims of 15-18 from people on FTE with 390s. I think the key is how they had it tuned/built. Also, I think they all had lower ratio rear ends. There was one thread in particular, try a search for rear ends, ratios, milage, or similar.

Tery

 

My '70 F100 Custom recently got ten mpg on a trip, but the carb is needing rebuilt. Back when I had a 2bbl Motocraft on it, 15 to 18mpg was the norm for my truck on the highway. It all depends on the tune of the engine. My '70 has a 390, 4bbl Holley, C6, 3.50 gears, 31x10.50x15's.

 

My 70' sport custom f-100 with a 390 and a 4-barrel 650 holley I got around 17 unless I kept my foot in it then it would sink to 12-13. By the way it had a C-6 a 203 full time T-case (Front driveshaft removed for highway driving) and 3:50 Gears.
Steve

 

Do you guys think that my highway mileage will go up to the 13-17 range even if it's this low in town.

Also, what can I do to get better gas mileage? I put a mallory ignition module in a couple of months back. Should I replace dist. cap and wires?

 

4.10 gears, 255/70r16 tires. need I say more. I installed a 90 gallon gas tank in the bed. Now I don't stop as often. It just hurts when I stop.

I suspect 7-10 MPG is about what I see. But my engine is spinning at bout 3400 RPM on the highway. This may be where 7-11 got the term "Big Gulp" :-)

 

I forgot to mention my engine spins roughly 2500rpm @ 55mph. My engine has dual exhaust, but with the factory manifolds. You've got the right gears for decent mileage.The secret is tuning. Play with timing, fuel mixture, tire pressure, and everything else that may affect mileage. I tune for a few hours, and drive for a few days. Then I tune for a few more hours. Drive a few more days.
I have two timing lights, but neither of them get near my truck. I time by ear and seat of the pants. I use a vacuum guage to adjust my idle mixture. Don't forget to try different jets in your carb, too.
Good luck!

 

Thanks for all the info!

I was just wondering if you guys have any tips on tuning the carb for better economy, and if could retard or advance my timing for better economy? It is set at 6 degrees BTDC right now (I think).

I need to get this thing to about 12mpg or more. I don't want to have to get rid of it..I love this truck, and it runs so good!!!

 

Advance it to 12BTDC. If it pings back it off but I doubt it will. This can add a good amount of power. It did for my 360 anyway. It was real real sluggish at 6BTDC.

 

I can get 12mpg out of my truck with 4.56 gears and 33x12.5.16.5 tires on the highway. So I think 12mpg shouldn't be too tough to get with low gears. A good tune up, and a properly adjusted carb. I got an extra 2 mpg on the highway when I put on my 4 bbl.

 

You said 390, but I thought you might want to know how a 360 (very similar) does. Mine spins about 2400 rpm going down the highway at 55 mph. Here are the mileage numbers in various road conditions: Highway: 10 mpg. In town: 10 mpg. Summer: 10 mpg. Winter: 10 mpg. Extra heavy on the gas, doing burnouts at every stoplight and such: 10 mpg. Driving for mileage: 10 mpg. See a trend forming? heh.

 

Thanks Rusty, I needed that.

 

Sad part is, its true!

 

I can totally believe that. worked on a farm for an ole boy that had a 73-79 Ford with 460 in it. That thing got 9 Miles to the gallon no matter what. I could pull a 20' stock trailer full of cattle and it got 9 miles to the gallon. Just running down the Highway no load 9 miles to the gallon. The best thing about it though. Even if it was loaded with cattle it pulled the same, as hard as you wanted it to. LOL Kinda cool though.


Scotty