[Jeepster Tim]

I have a 1981 CJ7 that had a 4 banger and 4 speed manual trans. I am trying to figure out what gearing i have so i can figure out what speedo gear to use. I think it may be a 410 but not sure..Thanks

[78treehugger]

It's hard to tell. Do you know for sure they are factory? There are a couple ways to find out. Pull the cover and count the teeth or mark the driveshaft and rotate the tires and count rotations. I prefer to just pull the covers and count. Gear lube is cheap insurance.

[foggybottombob]

Wrong way to go about finding a speedo gear. All you need to pick a speedo gear is a true speed vs indicated speed. So either borrow a GPS or have someone drive beside you at some steady speed and record the actual speed vs the speed indicated on your speedometer. Use that ratio to multiply by or divide into your speedo gear tooth count. If you want the speedo to read faster than it does then you want a smaller gear. If you want the speedo to read slower then you want a larger gear. It's 7th grade arithmetic.

[Jeepster Tim]

I don't believe it has been regeared. I just changed the gear oil and really don't feel like droppin the pan again... I thought that there might be a standard match with the 4 banger, the SR-4trans., and the AMC 20 rear axle.

[bigjoe]

If you think the gears were not changed yoy can look for the letter stamp on the axle shaft that gives you the axle ratio. For the Model 20 rear end look on the pass side of the diff. You may have to clean and remove some crud but on the rear axle on the side that the cover is on the factory stamped a letter (example: A, BB, D, DD) that is the ratio for that axle. If you can't find it you could pop off the cover count the teeth or get the ratio of the gears themselves. Or you could do the math as foggy said. Or finally when the police comes to your window and says "you were do x in a 55."
I vote for either option one or two first. Goodluck.