How to tell axle ratio

Many people ask how they can determine axle ratio on their rig or at a wrecking yard. If you have a Dana axle, it is easy. There is a verticle tag bolted to the diff cover

http://i649.photobucket.com/albums/u...psc254617b.jpg

The top number is the Dana part number for the axle. The bottom row has two numbers. The left one reads 4 10, meaning it is a 4.10 ratio. The bottom right hand is the tooth count for the ring and pinion gears 41 - 10

If there isn't a tag on your axle, isn't it possible to ball park a gear ratio if you spin a wheel once and count the number of driveshaft rotations? Or some combination of that?

Yes. If it is an open diff, jack one wheel up and count the number of wheel rotations for one exact driveshaft rotation and divide by 2. If locked, jack both wheels up and count the wheel rotations and do not divide by 2