Rear diff swap
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Cross-Country
Joined: May 2011
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Rear diff swap
Hello! I have a 64 f100 292 mt 3 spd as my daily driver and it's every bit Eco-friendly at 11mpg! I am a college student, and with the price of gas in my neck of the woods at $4.20 it's harder to keep driving my old' pik em up truck..So my 64 is geared to 3.89 (rear axle code-12) and I have out back a 66 f100 geared to 3.31(c1)...by passing the fact it's going to take some time and effort to swap the rear end, would I be able to see a difference in gas mileage to make it worth it? I'm not worried with losing the power with going to taller gears. Also if you guys have any other ideas like electronic ignition etc.. on how to increase gas mileage that would be wonderful! I know it's never going to hybrid fuel efficient but it be great to get a little closer!!
Thankyou Aaron
Thankyou Aaron
Ford Parts Specialist
Joined: Nov 2006
Posts: 88,826
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From: Simi Valley, CA
Two totally different rear axles:
12: Ford 9" / 3.89-1 / No Limited Slip.
C1 = Spicer/Dana 44 / 3.31-1 / Limited Slip.
LATimes said today that the price of gas will soon drop, but how much and for how long?
These trucks have the aerodynamics of a brick. A gallon of regular gas back then was 25-30 cents, so...no one gave a hoot about MPG.
You can spend a grand, you can spend 10 grand to improve MPG, but all you will be doing is throwing your money away. The paltry increase in HWY MPG isn't worth the trouble.
Expect no more than 8-12 CITY. You might get 17/18 HWY if your drive like Uncle Fudd (55 MPH).
12: Ford 9" / 3.89-1 / No Limited Slip.
C1 = Spicer/Dana 44 / 3.31-1 / Limited Slip.
LATimes said today that the price of gas will soon drop, but how much and for how long?
These trucks have the aerodynamics of a brick. A gallon of regular gas back then was 25-30 cents, so...no one gave a hoot about MPG.
You can spend a grand, you can spend 10 grand to improve MPG, but all you will be doing is throwing your money away. The paltry increase in HWY MPG isn't worth the trouble.
Expect no more than 8-12 CITY. You might get 17/18 HWY if your drive like Uncle Fudd (55 MPH).
New User
Joined: Aug 2010
Posts: 23
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From: Pleasanton
how much highway/city driving are you planning on doing?
i'm in a similar situation, and recently changed from 3.89 to 2.75 but only because i will be driving mostly highway to and from college next year.
if you are going to be doing any significant amount of city driving, the taller gears will mean your engine has to work harder to get going from a stop. this pretty much kills any mpg gains you'd make simply because of stop and go traffic around town. even with a 352, i noticed a significant loss of power with the gear change. it really purrs at freeway speed, but is a little too tall for around town driving.
if i had to do it over, i would find a ratio somewhere in-between the two.
that, or you could always look for an OD unit, but like numberdummy said, you can throw a lot of money at it and see not much gain in mpg.
aerodynamics like a brick
i'm in a similar situation, and recently changed from 3.89 to 2.75 but only because i will be driving mostly highway to and from college next year.
if you are going to be doing any significant amount of city driving, the taller gears will mean your engine has to work harder to get going from a stop. this pretty much kills any mpg gains you'd make simply because of stop and go traffic around town. even with a 352, i noticed a significant loss of power with the gear change. it really purrs at freeway speed, but is a little too tall for around town driving.
if i had to do it over, i would find a ratio somewhere in-between the two.
that, or you could always look for an OD unit, but like numberdummy said, you can throw a lot of money at it and see not much gain in mpg.
aerodynamics like a brick
Posting Guru
Joined: Jul 2009
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From: Ventura, CA
I've got a 302/C4 with 2.75s right now and the best I've gotten was right aroun 15 mpg on a tank of all freeway driving doing 55 because the other ppl I was with we're pulling a trailer. Normal for me is about 11-13 with slightly more freeway driving than city driving. So ya changing the gears may help but it's a cost vs benefit dilemma.
I would love to switch over to a ratio in the ball park of a 3.75 behind an AOD to help with low end power without losing too much on the top end but unless I can find an axle for cheap with disk brakes it's simply not cost effective
I would love to switch over to a ratio in the ball park of a 3.75 behind an AOD to help with low end power without losing too much on the top end but unless I can find an axle for cheap with disk brakes it's simply not cost effective
Fleet Owner
Joined: Jun 1999
Posts: 24,552
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From: Creedmoor, North Carolina
Aaron, Since you already own the 66, swapping the complete rear end might work. Brakes would be the concern, you might need to replace wheel cylinders & brake shoes, not a huge cost. You could take care of that pre install so it wouldn't slow you down when you did. Check the U-joint connection to see if they are the same.
Don't know how long your trips are, but picking up 1 or 2 mpgs can help. I don't have experience with the 292 but I don't see why it wouldn't handle a 3.31 ratio.
I ran a 2.75 behind a 352, it was a dog on take off but once up to speed it did ok. I swapped it to a 3.25 and liked take off much better, but like you say the cost of gas hurts over & over.
Get a GPS mph app for your cell phone and see how close or far off your speedo actually is going down the road. That can throw your mpgs off too.
John
Don't know how long your trips are, but picking up 1 or 2 mpgs can help. I don't have experience with the 292 but I don't see why it wouldn't handle a 3.31 ratio.
I ran a 2.75 behind a 352, it was a dog on take off but once up to speed it did ok. I swapped it to a 3.25 and liked take off much better, but like you say the cost of gas hurts over & over.
Get a GPS mph app for your cell phone and see how close or far off your speedo actually is going down the road. That can throw your mpgs off too.
John
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Cross-Country
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Thankyou guys for all your replys! I swapped the rear ends out last night and they were a perfect match! Only thing different were the lengths of the e brake cables. Makes sense since the 66 a long bed. I have yet to pull out of the shop as today it needs a a good rear brake drum overhaul! So hopefully in a few hours well take it for a spin!
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Ford Parts Specialist
Joined: Nov 2006
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From: Simi Valley, CA
Makes sense The cables are the same whether it's a short or long bed, as you can see:
1) C1TT-2853-AA .. Front Parking Brake Cable-80 3/4" long / 1961/64 F100/250 2WD
2) C3TZ-2A635-D .. Right & Left Rear Parking Brake Cables-67 7/16" long / 1963/64 F100 2WD
The front cable attaches to a bracket located on the crossmember. The cable adjustment rod attaches to the bracket then to an equalizer bar that the two rear cables attach to.
The rod has two nuts, one on either side of the equalizer bar. All you gotta do is re-adjust the cables.
Postmaster
Joined: Oct 2008
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From: Renton, WA
Thankyou guys for all your replys! I swapped the rear ends out last night and they were a perfect match! Only thing different were the lengths of the e brake cables. Makes sense since the 66 a long bed. I have yet to pull out of the shop as today it needs a a good rear brake drum overhaul! So hopefully in a few hours well take it for a spin!
That said I'm sure there is quite a bit of cost effective improvments that can be made. The best place to start is ignition. Not only upgrades to electronic and more powerful. But more important and almost free timing adjustments. A tuned vacuum advance off manifold vacuum will do alot. I real good tune of intitial, not just spec, and total will do even more.
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Cross-Country
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Everything went great with the swap! Runs great! Rpms are running lower as Im cruisng 60mph! And while I was at it I replaced the rear drums, shoes, cylinders, and all the hardware. E-brake cables went on no problem as well. I did hook up the driveline, U joints wrong and and when I took of down the road for a test run everything shook really good. After a quick call to my dad in canada I realized it was out of balance because I didnt realize how important it was to make sure the u joints were in the proper position! It was a nice little weekend warrior mechanic job! It also was leaking out of the pinion seal and I did not get a chance to fix that today. Any advice on when I do that? Do I need a special tool to pull the pinion out?
Thankyou
Aaron
Thankyou
Aaron
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