Lower gear ratio for better highway mpg's?

[tmcalavy]

My 69 F250 with 360 V8 has a code B4 rear end, a Dana 60 with 4.10 ratio. Got to wondering if a 3.50 or 3.0 ratio would improve my highway mpg? I know the truck is an aerodynamic brick, but would this help push my highway mpgs to near 14? Right now I'm getting 10 to 11 mpg in town driving and about 12 on the highway at 60 mph. Don't have a tach, but I'm guessing rpms are around 2,500+ at that speed. Second question is how do I tell if I have an 8-inch or 9-inch differential. I think F250's had the 9-inch, but not certain.
My ultimate goal is to replace the tired 360/C6 with a 300 I-6/C6 combo...working slowly on that one, tranny's are abundant but can't find a good I-6 that isn't whipped...yet.

[forrestbump]

Your Bump should have a full floating Dana 60 rear Axle housing in it unless it's been monkeyed with. Your mileage will probably not change much if you change your axle ratio due to the weight and lack of aerodynamics of the truck. Mine, for example, gets 8 MPG with a 390, C6 and open 3.73 in an F250.

Your best bet is the 300 I6. Just keep looking and you WILL find a good running 300 I6... they're out there.

[ScreaminZ]

My 69 F250 with 360 V8 has a code B4 rear end, a Dana 60 with 4.10 ratio. Got to wondering if a 3.50 or 3.0 ratio would improve my highway mpg? ... Second question is how do I tell if I have an 8-inch or 9-inch differential. I think F250's had the 9-inch, but not certain.

Your axle is not an 8 or a 9 inch rear. It's a Dana 60, which I believe is more like a 9.75" gear. Anyway, lowest (numeric) gear ratio it will go to is 3.54.

[survivorman]

You can get a dana 61 with a 3.07 gear. They look just like a 60 though.

You will need to do some bolt-ons for the 360 to be happy with it.

[tmcalavy]

bolt ons? on a differential?...pray tell

[jzjames]

Think Inline Six. Its what you want. Ive heard of people swapping in fuel injected inline sixes and having very good mpg.
4.10 is tough. I would suggest 3.70 or 3.50 rear differential, and that would be 9 inch.
Someone will probably disagree.

[tmcalavy]

Thanks guys. I'm still lookin for a good six...may get a chance to look at another one today. Plan is to acquire a six, with or without a C6 attached, and root up a PS bracket and radiator to make the swap idea complete. I can change the rear gears to 3.54 ratio for about $120, so that's a possibility too...before or after the swap. Meantime, I have to pull the valve covers and check gaskets and valve stem seals on the 360...it eats oil, like 1/2 quart a week in town driving, and 2-3 quarts on the highway per 250 miles. Curious thing though, it doesn't smoke at startup or when rollin down the road...just drinks it up. Compression is good on all eight jugs, so it's got to be somewhere inside the motor. I also have a C4 that I can put behind the six, but that will require some kind of driveshaft mod. Meantime, I am searching and planning..about a week from now I'll have time to start the process if a decent six shows up between now and then.

[survivorman]

bolt ons? on a differential?...pray tell

Electric fan, and dual exhaust to make sure you have enough torque.

The electric fan alone will give a a few mpg's back if your fan is direct drive. Some say 20 HP to drive a direct drive fan.

Also headers on a FE are worth about 30HP and 30TQ through out the whole rpm band.

Combined you'll be around 50tq where you need it to get a F250 moving.