Rear end change out
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Mountain Pass
Joined: Oct 2002
Posts: 149
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From: Metro Atlanta
Rear end change out
I have a 87 F150 4x4 my dad special ordered with a very low geared rear-end for driving on the beach for surf fishing. It gets about 10 -12 mpg, even on highway. I want to put in a replacement gear set to increase that mpg. how do I find out exactly what set I have in there now & where's the best wayplace to get a replacement. I can always go over to the local pull-it lot to take it out myself, but need to know how to find & decode the sets to make sure I get the right ones. I'll let you guys know what I have in it once I know where to look & how to decode.
TIA
TIA
Super Moderator
Joined: Apr 2004
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Your '87 probably has the Ford 8.8 integral carrier axle. There may still be a ratio tag on one of the bolts holding the rear cover on. Look for that and post what it says.
ID info:
Axle Identification Chart
ID info:
Axle Identification Chart
Laughing Gas
Joined: Dec 2007
Posts: 959
Likes: 8
From: Imperial, MO
In situations where you aren't able to locate an axle code, or don't trust it:
Place the vehicle on jackstands, make a mark on the driveshaft and on one tire, and count the number of driveshaft rotations necessary to make one full tire rotation. That will tell you your gear ratio (3 1/4 = 3.25:1, 3 1/2 = 3.5:1, etc.). For example, it will take slightly more than four driveshaft rotations to make one tire rotation if the carrier has 4.10:1 gears.
Place the vehicle on jackstands, make a mark on the driveshaft and on one tire, and count the number of driveshaft rotations necessary to make one full tire rotation. That will tell you your gear ratio (3 1/4 = 3.25:1, 3 1/2 = 3.5:1, etc.). For example, it will take slightly more than four driveshaft rotations to make one tire rotation if the carrier has 4.10:1 gears.
Post Fiend
Joined: Mar 2002
Posts: 19,402
Likes: 38
From: Bonita Springs FL
In situations where you aren't able to locate an axle code, or don't trust it:
Place the vehicle on jackstands, make a mark on the driveshaft and on one tire, and count the number of driveshaft rotations necessary to make one full tire rotation. That will tell you your gear ratio (3 1/4 = 3.25:1, 3 1/2 = 3.5:1, etc.). For example, it will take slightly more than four driveshaft rotations to make one tire rotation if the carrier has 4.10:1 gears.
Place the vehicle on jackstands, make a mark on the driveshaft and on one tire, and count the number of driveshaft rotations necessary to make one full tire rotation. That will tell you your gear ratio (3 1/4 = 3.25:1, 3 1/2 = 3.5:1, etc.). For example, it will take slightly more than four driveshaft rotations to make one tire rotation if the carrier has 4.10:1 gears.
That method will work if you have a limited slip but even then he forgot to tell you to put your transmission in neutral.
What you need to do is jack one wheel off the ground, put the truck in neutral, and spin that tire around twice. The number of driveshaft revolutions will be your gear ratio.
Laughing Gas
Joined: Dec 2007
Posts: 959
Likes: 8
From: Imperial, MO
With an open differential, what's going to happen if you follow this advice is one wheel will spin one way, and the other will spin the opposite way. The driveshaft won't move at all and you'll be no closer to an answer.
That method will work if you have a limited slip but even then he forgot to tell you to put your transmission in neutral.
That method will work if you have a limited slip but even then he forgot to tell you to put your transmission in neutral.
I assumed that placing the transmission in neutral would be self-explanatory, since you're attempting to rotate the driveshaft. The situation you described would only occur if you attempted to rotate the tire on an open diff with the transmission in park/gear.
As an addendum to my previous post, if your axle has an open carrier (only one wheel spins when you rotate the driveshaft), then yes, count the number of driveshaft rotations necessary for TWO tire rotations.
Fleet Mechanic
Joined: Oct 2010
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From: Delhi, Iowa
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Post Fiend
Joined: Mar 2002
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From: Bonita Springs FL
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Laughing Gas
Joined: Dec 2007
Posts: 959
Likes: 8
From: Imperial, MO
You're talking about turning the TIRE. I'm talking about turning the DRIVESHAFT. Two very different outcomes occur. First of all, no I don't attempt to turn the DRIVESHAFT while the transmission is in park/gear, as I'm just not quite strong enough to overcome the park pawl with my bare hands. Secondly, if you turn the DRIVESHAFT while the transmission is in neutral, an open diff will either spin one tire or both tires, however they will not spin in opposite directions. Now if you hold the DRIVESHAFT in place by placing the transmission in park/gear and spin one TIRE on an open diff, yes, the other tire will spin in the opposite direction.
I can go on, but is that enough of a writeup on this extremely simple procedure?
I can go on, but is that enough of a writeup on this extremely simple procedure?
That made my night.
Laughing Gas
Joined: Dec 2007
Posts: 959
Likes: 8
From: Imperial, MO
Exactly! I was referring to turning the driveshaft by hand, you were referring to turning a tire by hand. Two different situations.
Senior User
Joined: Feb 2011
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you could pop the cover off and count teeth (on both the ring gear and the pinion gear)... your diff probably could use new fluid in it any way...
also gearing for fuel milage takes a while to make a return on the investment... most shops will charge $800+ per axle to swap (out where i am $1000+) to make a few more miles per gallon... it would take quite a bit of time to make a return on fuel savings...
im guessing by your original question you probably dont know how to change gears yourself.
even pulling gears out of a junker truck you still need to set the back lash and all that correctly, and you are using junker parts
but if you REALLY need to know your gear ratio the BEST way to do it is to count the teeth
also gearing for fuel milage takes a while to make a return on the investment... most shops will charge $800+ per axle to swap (out where i am $1000+) to make a few more miles per gallon... it would take quite a bit of time to make a return on fuel savings...
im guessing by your original question you probably dont know how to change gears yourself.
even pulling gears out of a junker truck you still need to set the back lash and all that correctly, and you are using junker parts
but if you REALLY need to know your gear ratio the BEST way to do it is to count the teeth
Post Fiend
Joined: Mar 2002
Posts: 19,402
Likes: 38
From: Bonita Springs FL
