gear ratio

billfish

billfish

Senior Member
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?
 
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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.
 
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
 
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 ****** 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.
Toby.
 
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
 
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.
 
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.
 
billfish said:
Thanks to all for the good information--I'll check into the 51 to 55 rear.
Click to expand...
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
 
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.
 
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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