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Old 03-26-2020, 03:01 PM   #1
billfish
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Default gear ratio

looking for a way to lower the RPM at cruising speed by changing the stock differential gear ratio on my friends 1951 ford convertible. It's powered by a 302 with C4 auto trans with no overdrive. With the stock rear at 3.9 gear ratio it runs 3,000 RPM's at 50 miles an hour. He's not interested in quick starts just a smother ride at cruising speeds.

Is this something that can is done or can be done reasonably without going for a new transmission with overdrive?

Last edited by billfish; 03-26-2020 at 03:03 PM. Reason: more information
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Old 03-26-2020, 03:07 PM   #2
JSeery
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Default Re: gear ratio

Look at a speed vs rpm calculator. You can change tire size or rear axle ratio.
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Old 03-26-2020, 03:14 PM   #3
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Default Re: gear ratio

I believe the rear gear from an automatic trans car is about a 3:30 ish.
That's about the best your gonna' do with out an overdrive.
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Old 03-26-2020, 03:37 PM   #4
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Default Re: gear ratio

What 42Merc says , is probably the easiest and best way to achieve your goal. I think that the 49-51 Ford had a drop out center , If you can find a used rear diff, or just the drop out center from one of these that was originally an automatic car, I believe the rear ratio was 3:31. If I am wrong about the 49-51 Fords having the drop out, I know the 52-53 Fords did, as I have changed many center sections on those. The whole Diff may be able to be swapped in, but you would have to measure to be sure. I have not worked on one of these early Fords for over 30 years, and none around here to look at and check for you. good luck.---Jim
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Old 03-26-2020, 03:37 PM   #5
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Default Re: gear ratio

I have instructions on how to put in a 70's V8 Maverick or Comet automatic rear end in a Shoebox. It would give you a rear end ratio of 2.79. The standard tranny rear end would be a 3.00. The V8's have a 5 on 4 1/2 bolt pattern. email me at tpeterik1@wi.rr.com. I can send it email or snail mail.
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Old 03-26-2020, 04:26 PM   #6
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Default Re: gear ratio

There’s a company that advertises in Hemings that has automatic ratio rears for ‘49- ‘51 cars . It is a drop out center section so changing would be easy. If you want that info I’ll send it to you tonite...... Mark
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Old 03-26-2020, 04:35 PM   #7
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Default Re: gear ratio

Here's from Rumble seat. Boy he new his stuff.
Attached Files
File Type: pdf formula.pdf (38.6 KB, 71 views)
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Old 03-26-2020, 07:56 PM   #8
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Default Re: gear ratio

The ad lists over fifty 49-50-51 cars he’s parting out. For an automatic rear it would need to be a ‘51. Contact info is: Chris 610-442-2187 afternoon, PA.
Im going to post this on your thread too, for others.
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Old 03-26-2020, 10:25 PM   #9
Alaska Jim
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Default Re: gear ratio

flatford8, Thanks for pointing that out about the rear for an automatic having to be from a '51. I forgot that that was the first year for the automatic.
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Old 03-27-2020, 10:27 AM   #10
deuce5wndw
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Default Re: gear ratio

1951-55 automatic cars are the same except station wagon and sedan delivery. 3:30 ratio
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Old 03-27-2020, 10:46 AM   #11
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Default Re: gear ratio

Quote:
Originally Posted by deuce5wndw View Post
1951-55 automatic cars are the same except station wagon and sedan delivery. 3:30 ratio
Deuce is correct. My avatar has a '55 Ford rear end in it. The rpms dropped by 1000 at 60mph from 4:11s that was in it.
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Old 03-27-2020, 12:21 PM   #12
billfish
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Default Re: gear ratio

Thanks to all for the good information--I'll check into the 51 to 55 rear.
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Old 03-27-2020, 03:53 PM   #13
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Default Re: gear ratio

Quote:
Originally Posted by billfish View Post
Thanks to all for the good information--I'll check into the 51 to 55 rear.
If you happen to find a '55 or '56 Ford pumpkin that you want to use, make sure you get the axles that go with it. I know in '56 the splines on the axles have a different count than the '51 and I suspect that '55 is the same situation. The "56 axles are the same length as the '51 so the '56 axle/pumpkin combo will fit the '51 housing with no problem.

Ken
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Old 03-27-2020, 07:10 PM   #14
Ol' Ron
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Default Re: gear ratio

I used a78 ford rear from a C4 car in the junkyard. It had 3.07 ratio. I used it in a 52 PU with a 302 and c4 cruise was in the mid 2k around 65/70. It was practically a bolt in.
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Old 03-29-2020, 12:18 AM   #15
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Default Re: gear ratio

I put 1954 Ford 3.54 center section and axles in my '51 coupe many many years ago. '54 had the finer spline axle so both center section and axles had to change.

Dave
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