Gear ratio question
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Joined: Feb 2006
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Gear ratio question
Hello all, this is my first post. I've already gotten some good info here, so I had to sign up. Everyone seems pretty knowledgable. So, here's my question: Just got an 84 F150 straight six, 4 speed. I drive about 20 miles a day between work and home. I just figured I'm getting 10mpg, HIGHWAY!! My axle code is 18, so I guess I have the 3:50s? Anyway, can anybody give me some advice on replacing my rear end with something more like 2:47? Acceleration really isn't an issue, fuel economy is. What should I do? I don't want to have to drive 50 or 55 just to save gas. Any help would be greatly appreciated. Thanks.
Brad
Brad
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Joined: Jun 2004
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From: Pelahatchie, MS
The OD tranny will definitely help on mileage over the 4spd granny low. Tire size will affect it. How is the engine running? 10 out of a 6 is a little on the low side. 3.08 or 2.47 gears will help mileage. I get 10 out of my V8 4x4 with 3.50 gears and an automatic.
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From: Connecticut
Originally Posted by davidsca
Hey DM,
that seems like a nifty book to have on hand. What is it?
DS
that seems like a nifty book to have on hand. What is it?
DS
Just something I put together. I took info from alot of differnt sources to make my own. It includes electrical,vacumm,heating and cooling,fuel system,brakes,front and rear end break down diagrams, axel door code list with mating engine with manual or automatic trannies to give vehicleGCRW. Some of which has been put into my gallery. With more to come later as I catagorize it.
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New User
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Gear ratio question
I have the regular 4 speed, no OD. Not much mechanical knowledge, either. Motor runs well, just too many revs at 65. What exactly is involved in changing the gears from 3:08s to 2:47s? Where do I start?
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Brett Foote
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From: GANS
2.47s will cause you to lose gas if you drive stop and go. Being in kansas, they'd probably do you well on the highway. I had 2.47s (and live in western pa) in my f150, and never got better than 12, b/c I was always on the gas to go up hills.
easiest thing to do is goto the junkyard and grab a 2.47 8.8" or 9" axle and swap them. that is the easiest way. the only ones I've seen with 2.47s are 2wd f150s with 3 on the tree.
if you have a 4wd, it will be costly. I've never seen a 4wd with a gearset lower [numerically] than 3.08.
easiest thing to do is goto the junkyard and grab a 2.47 8.8" or 9" axle and swap them. that is the easiest way. the only ones I've seen with 2.47s are 2wd f150s with 3 on the tree.
if you have a 4wd, it will be costly. I've never seen a 4wd with a gearset lower [numerically] than 3.08.
Postmaster
Joined: Dec 2004
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From: Maple Valley, WA
I just pulled a 2.75 geared 9" out of my 81. It originally had a 6cyl/4spd with overdrive. The thing got lousy gas mileage and would lug. It might work better for you because you dont have an overdrive tranny. I now have a 302 with the same tranny and 3.50:1 gears, it avgs 15 with plenty of in town driving. If I were you, Id look around for an overdrive transmission, youll get the benifits of 3.50 gears when you need them but also gain some mileage with the final drive ratio.
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Why will I lose gas in stop and go? And do I need the entire axle? Can I not just buy what I need? I saw a 2:47 rack and pinion on ebay for like 50 bucks. Is there more to it than that? And is there a shorter gear ratio available so I don't waste gas in town?
Postmaster
Joined: Dec 2004
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From: Maple Valley, WA
With a longer gear ratio, the reason you use more gas in town is because it takes more gas to get the truck moving with those gears. Stop and go will kill your gas mileage with those gears and the truck will feel gutless as well.
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From: GANS
Originally Posted by BradfordH
Why will I lose gas in stop and go? And do I need the entire axle? Can I not just buy what I need? I saw a 2:47 rack and pinion on ebay for like 50 bucks. Is there more to it than that? And is there a shorter gear ratio available so I don't waste gas in town?
You can buy a 2.47 ring and pinion, however, you'll have to have them professional installed (not something an average shadetree mechanic can do). If you find the whole rear axle out of a truck, you can just unbolt the current axle and replace it.