Differential gear question
Senior User
Joined: Jul 2012
Posts: 295
Likes: 1
From: Centerpoint, IN
Figure out what tranny you have and the gearing and then put it all into this:
Gearing Calculator
Just put in your current tire size and gearing and then open another tab and play around with the axle gear to see what speed idle will be with your different gearing options.
Gearing Calculator
Just put in your current tire size and gearing and then open another tab and play around with the axle gear to see what speed idle will be with your different gearing options.
Lead Driver
Joined: Jul 2009
Posts: 6,477
Likes: 19
From: Sheffield, AL
My son's 1984 F150 with 4.9L engine; 4 speed manual O.D. and 2.47 rear gears [original] - used to get 26 MPG @ 60 mph, but had very sluggish acceleration.
Switched to 3.55 rear gears and lost 2-4 mpg @ 60 mph, but was much more fun to drive.
Going from 2.47 to 3.55 gears is a 43.7% increase in RPMs at any given speed. At 55 mph, the speedometer registered 79 mph, before correcting.
Switched to 3.55 rear gears and lost 2-4 mpg @ 60 mph, but was much more fun to drive.
Going from 2.47 to 3.55 gears is a 43.7% increase in RPMs at any given speed. At 55 mph, the speedometer registered 79 mph, before correcting.
Thread Starter
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Senior User
Joined: Nov 2012
Posts: 381
Likes: 0
Figure out what tranny you have and the gearing and then put it all into this:
Gearing Calculator
Just put in your current tire size and gearing and then open another tab and play around with the axle gear to see what speed idle will be with your different gearing options.
Gearing Calculator
Just put in your current tire size and gearing and then open another tab and play around with the axle gear to see what speed idle will be with your different gearing options.
Senior User
Joined: Jul 2012
Posts: 295
Likes: 1
From: Centerpoint, IN
In the box in the right put your tire size in one of the three formats, inches is easiest. Then you put in your axle ratio, redline(just put like 4000 or something it doesn't really matter). Then put in the first gear ratio of your transmission and scroll down and hit calculate.
Post Fiend
Joined: Dec 2007
Posts: 8,521
Likes: 16
From: Lost
If you intend to swap in an OD trans, I'd say go for something closer to the 3.50/3.55 range. Much more low end power, but in OD, it's like having a 2.73 axle.
OD trans and 2.73 axle will allow for great MPG, but you still won't be able to get moving without riding the clutch, particularly with a load....
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Thread Starter
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Senior User
Joined: Nov 2012
Posts: 381
Likes: 0
Not much to be gained.....
Kinda what I was thinking.....
If you intend to swap in an OD trans, I'd say go for something closer to the 3.50/3.55 range. Much more low end power, but in OD, it's like having a 2.73 axle.
OD trans and 2.73 axle will allow for great MPG, but you still won't be able to get moving without riding the clutch, particularly with a load....
Kinda what I was thinking.....
If you intend to swap in an OD trans, I'd say go for something closer to the 3.50/3.55 range. Much more low end power, but in OD, it's like having a 2.73 axle.
OD trans and 2.73 axle will allow for great MPG, but you still won't be able to get moving without riding the clutch, particularly with a load....
going off that chart thing.
with the gears that are in there not.it will run at 2200rpm at
1st 26mph
2nd 44mph
3rd 77mph
with the 3.55 gears it would run at 2200rpm
1st 18mph
2nd 30mph
3rd 53mph
runing on the high way it would be at 2700rpm at 65 and thats a bit high right.
with the 2.73 gears at 2200rpm
1st 23mph
2nd 40mph
3rd 69mph
wouldnt it still be better then wants in there now?
i mean first gear you can hit 45-50 2nd gear you can hit 80mph and 3rd gear as little to know power
Post Fiend
Joined: Dec 2007
Posts: 8,521
Likes: 16
From: Lost
Slightly better, yes. Easier to get moving with a load, barely.
In one post, you mentioned having a 4OD trans that you want to swap in. If that is still in the plans, then the 3.55 axle would be a better option.
With the 2.47 or 2.73 axle, you'd be better off swapping to a HD 4 speed like the NP435 or BW T18, as they have a "creeper" low gear. 2200 RPM would net *maybe* 5 mph......
In one post, you mentioned having a 4OD trans that you want to swap in. If that is still in the plans, then the 3.55 axle would be a better option.
With the 2.47 or 2.73 axle, you'd be better off swapping to a HD 4 speed like the NP435 or BW T18, as they have a "creeper" low gear. 2200 RPM would net *maybe* 5 mph......
Moderator
Joined: Jan 2001
Posts: 56,999
Likes: 2,747
From: Virginia
going off that chart thing.
with the gears that are in there not.it will run at 2200rpm at
1st 26mph
2nd 44mph
3rd 77mph
with the 3.55 gears it would run at 2200rpm
1st 18mph
2nd 30mph
3rd 53mph
runing on the high way it would be at 2700rpm at 65 and thats a bit high right.
with the 2.73 gears at 2200rpm
1st 23mph
2nd 40mph
3rd 69mph
wouldnt it still be better then wants in there now?
i mean first gear you can hit 45-50 2nd gear you can hit 80mph and 3rd gear as little to know power
with the gears that are in there not.it will run at 2200rpm at
1st 26mph
2nd 44mph
3rd 77mph
with the 3.55 gears it would run at 2200rpm
1st 18mph
2nd 30mph
3rd 53mph
runing on the high way it would be at 2700rpm at 65 and thats a bit high right.
with the 2.73 gears at 2200rpm
1st 23mph
2nd 40mph
3rd 69mph
wouldnt it still be better then wants in there now?
i mean first gear you can hit 45-50 2nd gear you can hit 80mph and 3rd gear as little to know power
Then the 5 speed overdrive trannies came out. You could then have your cake and eat it too. And now, 5 speeds were not enough, now they have 6 speed manuals, and 8 speed automatics. All to keep the engine in a narrow powerband so they can optimize it for fuel economy, emissions, and still be able to haul/tow something and go 80mph down the highway.
You will have to make a decision on what your most important priority is, or change the rearend and the tranny and then you can have it all. Sounds like a 87-up donor with a 5 speed is going to be your best bet.
Moderator
Joined: Jan 2001
Posts: 56,999
Likes: 2,747
From: Virginia
What are the 1st gear ratios of that tranny? It may not help you get what you want unless you change the rearend to a 3.50 or something like that at the same time.
Thread Starter
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Senior User
Joined: Nov 2012
Posts: 381
Likes: 0
when we swap the tranny we will swap the rearend gears to 3.55.