I have a 1995 Ford F-150 straight cab, 8ft bed, 300 I-6, 5spd (Borg Warner), 4x4 has 31x10.50/15's on it, and a 3.55:1 rear gear. I dropped from a solid 17mpg with the 235/75-15 to 14mpg with 31x10.50-15. I have a buddy with a F-150 EXACTLY the same as mine, but with a short bed. He installed a 3.73:1 gear and it bumped his milage up from 13-14 to a solid 16mpg.

I do alot of hauling/towing with my truck, so I am going to stick with my 3.53:1 rear ratio simply because I never use 5th gear now. My truck rarely sees speeds above 50mph, and I am turning just at 2,000rpms in 4th at 50mph. I don't shift into 5th untill about 55-60mph. so I keep my motor in the powerband a little better.

I tried shifting into 5th at 45-50mph between a fill up and my fuel mialge dropped to about 8-10mpg. If I cruise in 4th all the time I am getting a good 14mpg. Yea, it weird stuff I know, but it just works that way.

Cruising in a lower gear helps out ALOT!!! On a trip to Nashville from Knoxville I did 60mph with a 22' steel 5th wheel stock trailer and 3 horses (5 tons) in 5th gear and got 6mpg. I did 60mph on the way back to Knoxville from Nashville in 4th gear at 2600rpms the entire way with the same load and saw 7mpg. 1mpg doesn't seem like alot, but on a 190 mile trip with 5 tons behind a 300 I-6 it is. That 1 mpg difference saved me 4 gallons of gas, which came out to be about $12 because gas was $3.00/gal at that time. $12 is just under a hour of work for me.

 

I'd think you'd be fine as long as you aren't ******* it. You might pick up .5-2mpg if you upped to a 3.73:1. Do will deffinatly pick up some pep if you up a gear.

 

the only problem with getting a new rear axle gear is that well i don't know how much it would cost me to raplace the part, and i'd probably need a mechanic to do the job

 

You're probably looking at anywhere from $500-800 per axle, including parts. It would take quite a while to recover that in gas costs.

Jason

 

Yea... Right now, JEG'S is running a ring and pinion package with the lube, adheasive, shims, pinion nuts, crush sleeve, pinion seal, cover gasket, gear bolts, thread locker, marking compound, and of course the ring and pinion.

The 3.73:1 kit is $308.99 + FREE SHIPPING
And the 3.89:1 is $273.99 + FREE SHIPPING
The 4.11:1 kit is $268.99 + FREE SHIPPING

Installation is fairly easy, about a 4 hour job if you know what you doing or have a repair manuel right next to you.

 

are there any other ideas as to why my truck is getting the bad gas mileage other than the rear axle?

 

Well, if you don't want to change the axel, change your tires back to 235/75-15...

You said yourself that that's when you saw the change. We can't help you if you don't take the advice...

 

i am taking the advice i just wanted to know if there could be any oter possible problems

 

i'm a little confused, on the markers on our thruways with the cruise set at 60 i go 1 mile in 50 seconds so how do i figure how many mph i am traveling? thanks.

 

Here is an example of how that computation can be done:



Hope that helps!

-Chris

 

I don't know what your problem is, but I just made a 400 mile round trip to Branson, Mo. this past weekend & I got 16mpg with my 91 F-150 4x4. 302/e4od.
235/15 tires? I've noticed before that if I keep the tach at 1900-2000 rpm, that allows me to run around 73-74 mph, that's what I did on this trip. However, if I run it above 2200 rpm, I'll average about 12-13 mpg.

 

thanks that helps a lot, but its hard to believe i'm doing 72 traffic still passes me, but i know your'e right. i need to find a way to change my speedo or try to figure a speed not to exceed. thanks.

 

I checked my speedo with the new mile marker signs with the 1\10th mile segments they put up here in missouri & it's right on the money.

 

Check the articles at the top and also google for recalibrating a Ford pickup's speedometer, it's fairly straight forward. It involves setting your speedometer to program mode my pressing the Mode and Reset buttons in some pattern (I don't know which pattern), then marking your tire at 6 o'clock, and rolling the truck forward 3 revolutions on the tires and making the mark come back to the same spot, then pressing some buttons in the cab again. Like I said, pretty easy. But have someone watch the tires outside so you get it right the first time because you can only reprogram you speedo x number of times (I think it's 4-6 times).

-Chris

 

the thing with trying to stay under 2200 rpm is that on my truck there is no rpm gaude, only the speedometer and tachometer