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Old 11-06-2020, 08:02 AM   #1
satfix
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Default Rearend Question

I have a 52 F1 - After taking time to investigate and figure out what rearend is in the truck, I've determined it is the original Dano 44 with a 3.92 gear ratio. (found tag: W 47/12)

Here's my question - if I want to get better highway speeds and lower the RPMs on my motor, can I change this out for a lower ratio differential? I'm ok running 65 mph, but that equates to about 2900 RPM on my 239. I'd like to be able to run under 2000 RPM for highway.

I've researched changing the transmission, but it's not a can of worms, I want to open. It'll mean major modifications and I'd rather keep the 239 Flathead with 3 on the tree intact.

If I changed the gear ratio, it would be easier to take it back to stock in the future as well.

Thoughts? Am I chasing a rabbit or is this a possibility?

Thanks
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Old 11-06-2020, 09:32 AM   #2
flatford8
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Default Re: Rearend Question

Under 2000 might be a little low. There are a number of sites that have a formula to determine axle ratio- tire size - engine rpm. I don’t have one handy to link. I’m sure someone will post one shortly. You could try some taller tires to see how a higher ratio would work with the terrain in your area....... Mark
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Old 11-06-2020, 10:01 AM   #3
TJ
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Default Re: Rearend Question

You can use the 9" rear from a Ford pickup from 1957 to approximately 1972. They have the drop out third member which makes gear changes easy. The housing from these trucks bolts right into the F-1 pickup. You will have to modify the shock mounts which is an easy task. You can use any 9 inch third member from any Ford passenger car or pickup or you can install new gears into the center section with any ratio you desire.
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Old 11-06-2020, 10:13 AM   #4
satfix
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Default Re: Rearend Question

I found a calculator: My tires are 28 inches high - with the 3.92 rear end, I need to run 3000 rpm to reach approx 65mph. In order to do that at a comfortable RPM of 2500 RPM, I'd have to drop to about a 3.07 ratio - that would get me 67MPH. 2000 RPM would run about 55MPH.

Based on what I've read, it seems I'd have to do a complete rebuild (carrier change) in my rear end to accommodate the lower ratio. Jeep differentials have this ratio, but it sounds like my best bet is to simply sell this truck and find one that has the modern drive train already installed. Hate to 'butcher' this truck as it is such a nice example of an original 52.

Thanks for the lessons and guidance - as always!
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Old 11-06-2020, 11:35 AM   #5
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Default Re: Rearend Question

If you like your truck it would be much cheaper just to change the rear end rather than sell and look for another truck. Currently you know what you have and it can be fixed. Buying another truck could cost you a lot more money that just changing the rear end. Changing the rear end would not be butchering the truck.
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Old 11-06-2020, 11:40 AM   #6
Ross F-1
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Default Re: Rearend Question

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I went the 9" / 3.50 route. I briefly tried a 3.25 ratio, and it was a total dog. (Note I live up high, 5300+ ft, and plenty of hills/mountains). Even on flat ground with a 30 mph headwind I couldn't accelerate past 50. Swapped in the 3.50 and it is about perfect.

It won't hurt your engine to spin it up closer to 3000 RPM. And I'm not sure I'd want to drive at 65 mph in many places except wide open highway with no traffic, with stock brakes and suspension. These trucks just weren't made for that. I'd enjoy what you have, 55 isn't that slow.
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Old 11-06-2020, 11:41 AM   #7
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Default Re: Rearend Question

Quote:
Originally Posted by satfix View Post
Based on what I've read, it seems I'd have to do a complete rebuild (carrier change) in my rear end to accommodate the lower ratio.
In rear axle ratio lingo, a lower numerical ratio is referred to has a "higher" ratio. So say a 4:11 gear is a "low" gear and something like a 3:25 gear is a "high" gear.
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