rear end gear changes?????
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Cross-Country
Joined: Sep 2007
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rear end gear changes?????
I have a 86 F150 6cyl with a 3 speed with overdrive manual and 3.08 rear end gear ratios. The truck gets about 18mpg on average. I regularly pull a boat and when pulling I get about 15mpg. In overdirve the engine is running at about 1800rpms at 65mph. I would really like to have a bit more pulling power from a stop. Can anyone offer advice on making a rear end gear swap without killing the fuel mileage??? Anyone have a similar truck set up with say a 3.73 ratio?? I know that a lower ratio is going to raise my rpms at crusiing speed so I wonder how much that will affect fuel consumption.
Fleet Mechanic
Joined: Jul 2011
Posts: 1,916
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I have a 86 F150 6cyl with a 3 speed with overdrive manual and 3.08 rear end gear ratios. The truck gets about 18mpg on average. I regularly pull a boat and when pulling I get about 15mpg. In overdirve the engine is running at about 1800rpms at 65mph. I would really like to have a bit more pulling power from a stop. Can anyone offer advice on making a rear end gear swap without killing the fuel mileage??? Anyone have a similar truck set up with say a 3.73 ratio?? I know that a lower ratio is going to raise my rpms at crusiing speed so I wonder how much that will affect fuel consumption.
For your setup, and for the pulling you do though, I'd actually put in a set of 3.55 gears and have them put in a nice limited-slip clutch pack while they're in there too. The limited slip will make both tires catch when you lose traction, which can be very helpful when you're pulling a boat trailer with a boat on it up the loading ramp. The 3.55 gears will give you plenty of oomph off the line with your truck unloaded and loaded. It'll also make it a lot funner to drive too.
Gearing makes all the difference with a 300. A set of 3.55's will give the truck a lot more power, it'll be easier to start off hills and pulling, and 3.55's won't impact your mileage too bad. At most you should lose about 1 or 2 mpg. Depending on how you drive though, it could actually remain the same or get better.
Hotshot
Joined: Feb 2004
Posts: 11,697
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From: Marengo, Iowa
In order to get the limited slip, you have to swap the whole diff center section. The limited slip housing has an area cut for the clutches to sit in. You would need the pinion and diff carrier with ring gear. Then you have to set it all up to have the proper mesh depth to wear properly. If you don't then the axle likely won't last long. If you choose to only change ring and pinion, you will still have to set the depth properly.
Fleet Mechanic
Joined: Jul 2011
Posts: 1,916
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In order to get the limited slip, you have to swap the whole diff center section. The limited slip housing has an area cut for the clutches to sit in. You would need the pinion and diff carrier with ring gear. Then you have to set it all up to have the proper mesh depth to wear properly. If you don't then the axle likely won't last long. If you choose to only change ring and pinion, you will still have to set the depth properly.
Since I'm interested in a limited slip for my truck in the future (and I don't mean to hijack the op's thread), but how easy would it be to make the axle accept a limited slip? Would you need to get a whole new housing or could you modify it? Also, the ring and pinion have to be limited slip specific, or do you just need a new carrier?
If it's too much work to add in a limited slip, I'd much rather find a 9'' with LS and swap it in like I originally planned... of course I always have the option of adding a locker to the 8.8. But then again, if I'm swapping in a 9'', then why not a Dana 60? Then, if a Dana 60, why not do a solid axle swap on the front, and... and... and...
Aside from me... I think the OP will be very satisfied with a set of 3.55's.
Hotshot
Joined: Feb 2004
Posts: 11,697
Likes: 3
From: Marengo, Iowa
The absolute in LS is the diff itself, the rest interchanges. I would suggest reading upon the two types before digging in though. F150 and Explorers share the same gear components too, had to fix a 150 with LS here last summer, got a center from a 91 Explorer, worked perfect. The spider gears were broken on the 150. There isn't room to add anything between the spiders on either type, the clutches go behind the spiders. You then have to have a properly machined housing and spiders to be able to put in clutches.
Posting Guru
Joined: Jun 2004
Posts: 1,092
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From: Casa Grande
Yep, the LS replaces the open carrier you probably have now. But you might have a limited slip now, some did come with them.
I have the 300 with 4 speed overdrive and 3.55 gears on 235 tires. It does 18+ on the highway and around 14 city. So like said earlier, it wont hurt the mpg that bad.
I have the 300 with 4 speed overdrive and 3.55 gears on 235 tires. It does 18+ on the highway and around 14 city. So like said earlier, it wont hurt the mpg that bad.
Posting Guru
Joined: Jan 2012
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From: Dallas, GA
I had changed from a 3.31 (I believe that's what it was) all the way to a 4.11 and really didn't see much difference in the mileage. Sure did see a difference in the "get up and go" though. I wouldn't recommend that radical of a change but that 3.55 is always a great middle ground.
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Old School Hot Rodder
Joined: Feb 2006
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From: Exmore, VA
I put a LS center in my 10.25" rear, I kept the 3.55 gear as it always towed well. The easiest way might be to find an entire drum to drum 3.55 LS rear in a junkyard, Even if you just use it for parts you would still have everything you need. 302 4wd standard gear with the AOD was a 3.55. Get an axle code listing and look at the door post certification labels LS gears are identified with a letter/number, open gears with a number.
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Posting Guru
Joined: Jan 2012
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From: Dallas, GA
I put a LS center in my 10.25" rear, I kept the 3.55 gear as it always towed well. The easiest way might be to find an entire drum to drum 3.55 LS rear in a junkyard, Even if you just use it for parts you would still have everything you need. 302 4wd standard gear with the AOD was a 3.55. Get an axle code listing and look at the door post certification labels LS gears are identified with a letter/number, open gears with a number.
not to side track this thread but were you anywhere near where that fighter plane crashed today Bill? Hope everyone's ok
Posting Guru
Joined: Jan 2012
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From: Dallas, GA
Ah yes...bought my 78 F250 from a guy in the Portsmouth shipyards back in 92. My brother-in-law lived up in your parts. Fort Eustis? (spelling?)
Posting Guru
Joined: Jan 2012
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From: Dallas, GA
very cool...nice area. I only lived there for about 3 years there in Norfolk, was in the Navy. Spent some time in the Naval Shipyards over in Portmouth. My two oldest children were born in Chesapeake.
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