Need a quick & easy way to figure out gear ratio

I was doing to old car shuffle getting ready to move this project to the other side of the backyard for longer term storage, and I discovered it has a Sure Grip in it I believe. No tags. The yoke is 3 1/2" wide, is that small or large, I can't remember. Anyway I have the car jacked up with the wheels on it and I can turn the driveline. How do I figure out the ratio? I may have to "borrow" this gear for awhile if its a 3:55 It was probebly changed at one point. The car came with a 318/904. Would it be on the fender tag?Thanks

Turn the wheel once around and count the number of times the driveshaft rotates. 3 1/2 turns is a 3.55,3 1/4 turns is a 3.23 etc. That works if it's a sure grip. Both wheels will turn the same direction if it's a sure grip.

Thanks stumpy! Looks like it's a 3:55, but it's a small yoke. I may leave it in the shop for awhile and get it running so I can move it around under it's own power.It hasn't ran a in 10 years but it ran when it was parked.

There is nothing wrong with the small yoke.

any tricks for figuring out gear ratio on an open rear end?

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There is nothing wrong with the small yoke.

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My RR has the larger yoke, so it wouldn't be a quick swap, although they are both 489 cases. Would it be a big hassle to change out the yokes? I know those nuts don't come off too easy. I like the way this looks. The Challenger belongs to the guy who painted my car, I can pretty much do anything I want with it. Are plan was to put the roof on it , get it running, and sell it. I really don't need another money pit though.