2-Big Broncos 4x4 Technical White Papers

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Speedometer Gear Corrections

Question:

How can I figure out which gear I need to put in to get my speedometer back up to par. It seems a little off with these new gears in.

Craig Gauss
1989 Bronco XLT
2 1/2" Tuff Coutry Suspension Lift
33x12.5 Super Swamper SSR's
15x10 American Racing Baja Wheels
4.56 gears finally

Response:

Craig, the part guys at Ford can do this to some degree. I'm not sure if their info can accomdate 4.56 gears and larger tires than stock. If you know your stock gears and tires and are sure the speedo gear was never changed it's easy enough to figure out which gear you need based on what comes out of there. If you're not sure, my suggestion is have someone in a stock vehicle with an accurate speedo follow you to get an accurate reading and determine what percentage difference you have. You must look at the difference as a percent. Because that percentage variance has a direct relationship in speed. If your speedometer is reading faster than the other vehicles, you need to reduce the number of teeth in the speedo gear you currently have. The opposite will be true if your speedometer is slower.

This is how you can tell how much of a difference different teeth make. If you have a 20 tooth gear. One less tooth gear would decrease your speedometer 5% (1/20), two less teeth 10% (2/20). For one or two more teeth it would increase the speedometer 5% and 10%. This is as close as your going to get based on your actual difference. The most variation you would have is about 2% from actual, which isn't quite 2 mph at 70 mph. If you can find someone with a decent GPS you can also determine your speed that way. I don't know how accurate this is, however.

John -- pjblanchet3@yahoo.com
1992 Bronco XLT

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