New 8 1/4 rear

66dartgt said

to determine the axle ratio either the thing will have a tag as mentioned above or in most cases they are missing. If missing then rotate one axle one revolution and count the number of turns the pinion makes. 1 tire revolution to 2 3/4 pinion turns its a 2.7x gear etc.

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Just a note. If it is a open rearend. As in a non-sure grip make sure and rotate both axles exactly one full revolution not just one or you will get a false reading. It takes one guy on each side to do this properly. If you only rotate one axle and hold the other still you have to multiply the reading by 2. This due to the spider gear multiplication in the differential. If you have a sure grip you don't have to worry about this and figuring gear ratio is alot easier.