I am trying to figure out the gear ratio on a 1954 F100 rear end. I marked the drive shaft and one of the brake drums. For every 10 turns of the brake drum, the drive shaft turns 19.5 times. So I have a 1.95 rear?

 

First of all, look for a tag on the carrier plate bolts. If it is gone, then what you have to do is lock one of the rear drums, or have someone hold one drum from turning, while you turn the other. If you turn one drum, the differential will allow the other drum to turn backwards which will not turn the ring gear. Your truck will have a 3.92 if it is a six, or a 3.54 if it is a V-8 usually. Anyway those are in the ballpark.

 

NJ239Y

From what you have mentioned your diff is probably a 3.91. I myself have done this quite numerous times and it always works out to be on.
Just for sure I have some one help me in the count and turn the wheel 20 times and count the turns on the yoke.
If your still in doubt take off the cover and use some of the white correction liquid that they use to correct typo errors. Mark the pinion gear and count the number of teeth on it. Then do the same to the crown gear.
You will then know for sure, I know that this method works, the part that you did at first.
Check it again though by turning the wheel 20 times, and if it comes out to 39 turns on the yoke you know you have a 3.91.

 

Actually, I had the other drum on the ground with the turning brake drum on a jack stand. So with the angle the axel was on, the one on the ground should not have turned. But it must have. I will try it again with help this time, and I will turn the drum 20 times. I'll let you know what I come up with.

 

Well, it finally stopped raining, so I pulled out the axel from under my portable garage and had my wife count the yoke while I counted 20 turns on the drum. The yoke turned 39-1/8 turns, or 39.125 turns. So this is how they come up with the number 3.91?
Also, alanco (above) mentioned that a 3.91 or 3.92 is for a 6 cylinder. Is that always the rule, or did V8's have them too?

 

I know I'm a little late to the party here, but wanted to reply in case it helps anyone else.

I've got a 1954 F100 VIN F10V4R, so it had a 239 V8 and the rear axle is listed as a 3.92. I thought it was a 9" rear, but I guess those weren't used until later, so the best I can find so far is that it's an 8" rear, but even that's confusing.

Either way, I wanted to answer the question earlier in the post about a V8 model having a 3.91 (really 3.92?) gear ratio. Also, I attached a VIN decoder picture I recently ran across.

 

I think most standard trucks had 3.92 Dana 44 rear ends. That's what my 55 w/original 239 y-block had/has.

 

My '56 with 272 V-8 and Fordomatic has a 3.92 rear.