What gear ratio are you using?

[mustanguy]

Hey guys, My '71 f100 LWB, automatic, 390 comes with a 9" 3.25 open gears. My mileage is about 10MPG. I was thinking of swapping to a 3.00:1 ring and pinion i have to get a bit better mileage. What do you think? What gears are all of you running and what's the approximate mileage?

[JEF199]

Dout it would be a noticeable change. A good tune up and an accurate test will probably get you in the normal range of 12-15. For mine the key is to keep the speed down around 55mph.

You can try some taller tires, (narrow/highway tread)...

[GSequoia]

390 with a C6, 4.10:1 gears, and 9.50x16.5 tires on the drive wheels and I get between 7 - 9.5 with 87 octane fuel.

[sargentrs]

302, 3.03 std trans, 265/75-15 tires, 3.89 gears. Around 12 mpg.

[Fordnatic]

What RPM you running on the highway? With a built 390 and 3:50 gears and some low profile 15" tires I always ran 2400 at 60 MPH and got 12-14 mpg's.

[Alvin in AZ]

My '71 f100 LWB, automatic, 390 comes with a 9" 3.25 open gears. My mileage is about 10MPG.
I was thinking of swapping to a 3.00:1 ring and pinion i have to get a bit better mileage.
What do you think? What gears are all of you running and what's the approximate mileage?

You need to post your tire size, it can make a big difference in all this.

x2 what Jef said... :)
"...try some tall... narrow/highway tread [tires]"
...like 235/85-16's on 7" wide wheels.

And be sure to test your odometer and always calculate-in the percentage it's off by too.

-----------------

'75 F150
31.5" diameter 235/85-16E's with 3.50 axle and 360FE +T18
The 3.50 axle gives me a little less MPG than I got with the 3.25 but the power is worth it. :)

'91 Bronco
31.5" diameter 265/75-16E's with 3.55 axle and 351w +ZF
Plan to swap the gearing from 3.55 to 3.73 since it's got overdrive. ...got it all calculated ;)

Alvin in AZ

[mustanguy]

I plan on using 255 60 15 on the rear and 225 or 235 60 15 on the front. Radial T/As

[Alvin in AZ]

I plan on using 255 60 15 on the rear and 225 or 235 60 15 on the front. Radial T/As

So, what diameter are those? :)

Alvin in AZ

[mustanguy]

Well It turns out I'm not going to switch to 3.00 gear but when I get the BFGs They'll be a bit smaller diameter. so much for better economy.

[Joshpow]

302 c4 with 3.50:1. With a good tune and taking it easy on the go pedal I can get 15 or more mpg. If i play i get more like 10-12!

[cozadlance2]

3.73 Dana 60 and C6. by odometer (untested) and gas purchased I get 8.4 mpg w the 200,000 m 360 2v on 245/75/16s on an f250

[Mancar1]

68 CS, 390, C6, 4:10 Dana, A/T, 2V. 11 - 12 MPG.

[Racer Z]

Stock stock stock. 70 F250; 390 (fresh motor); C6; Dana 60; about 10 MPG. Not sure what the final drive is. It's stock, maybe 4:11.

Gearing up might actually get worse MPG, especially with a load. Cause you will be out of your power band.

[BobbyFord]

Stock 460 ( ), 3.50 gears, terrible mileage...