How to confirm differential ratio '40 PU

[jackstevens]

1st time poster so bear with me.

Bringing '40 Ford 1/2 ton PU back to life after 10+ years non-runner, stored in garage. Has after mkt wheels, P195/65r 14 tires which look cool but not practical for a driver. Banjo 3rd member marking is 9 34 indicating 3:78 ratio. Need to verify to get more appropriate size tires for tours.

In 3rd with one rear wheel off ground counting motor and wheel revs, got 1:90; same with other wheel free. Does that confirm 3:78? If so I will go to 15" wheels and bigger radials to get a decent at the road ratio.

A word to the wise - you cannot check out too many details on an old non-runner restoration prospect, especially one with older amateur restoration. Unfortunately PU was 1500 miles away so couldn't see it myself.

Seems like new snake under every rock we turn over! As owner of 6 flatheads earlier, I knew better but emotion of seeing solid straight sheet metal, Edelbrock heads, Mallory wires etc won out over rational thinking. For economics sake tho, buy one after a professional or expert/contentious person has done the hard parts - mechanical, wiring and body work/paint. Of course if you're very good at most of the crafts and like the feeling of doing it yourself, do a all nuts/bolts

body-off so you know all is right and enjoy it.

 

[ford38v8]

Jack, one wheel down spinning the other will double the turns, so yes, it sounds like your 1.9 was engine revs per wheel single wheel rev to confirm 3.78. If unsure, a more accurate confirmation would be to multiply the revs out to get a better read. (7.56 to 4 with one wheel down.)

 

[JM 35 Sedan]

The easiest way to determine rear gear ratio, that I have found, is to remove the threaded oil fill plug on the back of banjo housing center section, and count the number of teeth on the ring gear. This count can be related back to the gear ratios used in production for that particular model and year vehicle.

 

[Ggmac]

I love your truck ! Good luck with it and hope the bugs get worked out .

 

[Ford Freak]

Nice truck !! - F F

 

[zzlegend]

The easiest way to determine rear gear ratio, that I have found, is to remove the threaded oil fill plug on the back of banjo housing center section, and count the number of teeth on the ring gear. This count can be related back to the gear ratios used in production for that particular model and year vehicle.

That is the way I do it also. I just use a little white marker on the first one.

 

[Mart]

The method you used is as good as any. I do it like that, but just make the raised wheel do 2 revs. Driveshaft or motor turns (in top) then directly equate to the axle ratio.

Here's my little video I put on You-tube.

https://youtu.be/bkvp3Fq21ZM

Mart.

 

[Clem Clement]

Hey Jack,
We tried to look at everything and give you our best opinion. BUT, brakes frozen, tires flat, no time and... We did get the Mallory problem. Could not get under the pickup.
hope you are having fun, despite.
Clem

 

[JM 35 Sedan]

That is the way I do it also. I just use a little white marker on the first one.

ZZ, I do the mark on one tooth as well, with any type of marker I have handy. This method also works great when you need to check a rear thats been removed from a vehicle and may be laying on the ground at a swap meet. No tires, wheels or drums attached to turn or count turns with, and maybe no torque tube or drive shaft attached.

It's also not a good idea to "assume" the numbers stamped on the bottom of the banjo housing web/rib are always correct. Gear ratios are many times changed over the years for various reasons. Example: my cars will most likely have 37/9 (for 4.11:1) stamped on the banjo rib, while the gears will actually be 34/9 (3.78:1), or 39/11 (3.54:1). Maybe I'll attach some metal tags stamped with the correct numbers to make it easy for the next owners, or maybe they will just have to figure it out for themselves