[Fred T]

Here's an easier way. Get on the highway with a friend and a good watch with a second hand. While you drive at a steady 60 mph, have your friend check your speed between the mileposts with the watch. 60 mph is one mile in one minute. Then adjust your speed until you are covering one mile in 60 seconds and note the speedometer reading.

Then take your speedo gear to either a dealership or a tranny parts house, tell them that the gear in your hand gives you a reading of XX mph when you're actually going 60. They should be able to do the math and sell you the correct gear.