Hope this is not a dumb idea but it won't be the first for me if it is.
OK, My 51 F3 has the original 4 speed transmision. Would it be possible to install a rear axel with gears in the range that would make 4th gear act like an overdrive? Would driving around town in 3rd gear have a bad effect on the tranny? Am i way off base or is this doable? I'm not planning on using this truck for any heavy duty hauling. Just wondered if this was an easy overdrive idea.
Someone put me straight on this idea one way or the other. Maybe i can learn something .

 

I don't think they make a Gear Set with a high enough ratio for the stock rear end.
Best bet is to swap out the Stock Rear for a Ford 9", it's almost a direct bolt in.

 

On straight stick transmissions with 5th gear being OD, the 5th gear has a ratio of usually 0.50 to 0.67 to 1 or somewhere around that. I don't know what the ratio is for the 4th gear in your 4 speed, but it is not nearly tall enough for OD. I suppose you could conceivably use a very tall rear end ratio like something in the low 2s to make your granny gear (1st) usable and then have the rest of the gears as ODs but that would be just weird.

Vern

 

1. Nothing magic about overdrive - it just means that the final gear ratio in your transmission is more than 1 to 1. That is, that the driveshaft turns more than one turn for each revolution of the engine in high gear. Most overdrives are in the range of .7 to 1.

Mostly you do not care how fast your driveshaft turns, it is how fast your wheels are revolving that counts. That is set by your rear end ratio. If you have a 4.11 rear end, this means that the drive shaft will turn 4.11 times for every wheel revolution.

Therefore, if you have a standard transmission (1 to 1 high gear ratio), your engine will turn 4.11 times for each wheel revolution. With an overdrive transmission (.7 to 1 high gear ratio) and the same 4.11 rear end, your engine will turn 2.87 times for each wheel revolution.

So, if you install a rear end with a 2.87 ratio it will accomplish the same thing as installing a transmission with a .7 overdrive.

2. Truck type transmissions are built for heavy duty service in the gears - you can drive forever in any gear without causing any problems - I have spent hours in third gear towing trailers over the mountains. Running third in town is no problem. Some transmissions whine in the lower gears and that may bother you, but has no effect on the transmission.

3. Your question is not dumb, and your proposed solution would work. However, I do not think the ratio you would need is available for your stock rear end. Most people change out the stock rear for a ford 9 inch, which does have many different ratios available.

For better drivability I would chose a rear ratio between 3 to 3.25 to 1. With that ratio you should be able to drive the truck as you do now, and still get decent highway mileage.

 

Thanks guys for your input on my brainstorm. I'm more encouraged now to look into changing the complete rear axel to a later year axel with parts that are more available.
I have the drums off now on the F3 and its a pia trying to locate brake shoes for it anyway. Seals are leaking bad also and it needs new wheel cylinders. I could put that money into a 9in. with a more comfortable gear ratio for hyway use. Glad to know that the old 4 speed could take using 3rd gear for long periods of time. I don't mind the whining. Going to start watching for a deal on a rear axel now. Lots of old Ford trucks in our local papers. Maybe i'll end up with another whole truck for other parts too.
Thanks again.