how can you tell what size gears you have?

[Mr 5 0]

Originally posted by GodStang

It should say on a tag on your axle.

There's also a door tag with a code but if you have aftermarket (non-stock) rear gears, all that information is useless.

Use the 'marked driveshaft/one tire rotation' method to determine rear diff ratios, especially on older Mustangs that very often have aftermarket gears installed as this is a basic Mustang mod.