Speedo Calibration

Q: Speedo Calibration

My speedometer is off by about 5 to 10 mph. How do I correct this?

A: There are three ways to calibrate your speedo. do this.

For the first one, find out exactly how far off it is… try using the odometer to check how many miles you read when traveling 10 miles according to mile markers. You might get a ratio like 1.4:1 (for example) Next, pull the speedo gear and housing out of the trans. Count the number of teeth on the gear (driven gear). Also, count the number of teeth on the gear it mates to on the output shaft of the trans (drive gear). Take the number of teeth on the driven gear and divide by the number on the drive gear. Lets say you get 35/19=1.84, now multiply by how far off your speedo is (like 1.4) and you get 2.58 (in this example). This is what the final driven gear to drive gear ratio needs to be. You may not be able to get this ratio by just changing the driven gear since they are only made from something like 35 to 45 teeth. Also, if you change the driven gear by too much, you will also need to change the driven gear housing since they only house ranges of gears like 36-39, 40-43, etc. Since the housings cost about $25 and gears are only $7 or so, you may want to find a combination that keeps your driven gear in the same housing range that you have today.

The second way to do this is to use one of these formulas.
Drive Gear = (.0495835 * Tire Dia * Driven Gear) / Gear Ratio
OR
Driven Gear = (20.168 * Gear Ratio * Drive Gear) / Tire Dia.

And here’s a third way, call Ronnie Redd at Jerry Brown Chevrlolet in Buford, GA 770-945-4981. He’ll want to know Gear Ratio, Tire Size, type of Trans and then he can give you GM part numbers for the gears and housing you need.

When I had to do it, I actually used the first method to figure out what I needed, the second method as a sanity check, and the third method to get the GM part numbers so I could order them from my local dealer. Ronnie Redd was very nice and helpful, even though he knew I wasn’t local and wasn’t going to buy the parts from him.

BTW – Here are some of the GM Part Numbers

Drive Gears
17 tooth 1246221

Driven Gears
36 white 1359270
37 red 1359271
38 blue 1359272
39 brown 1359273
40 black 1342048
41 yellow 1362195
42 green 1362049
43 purple 1362196
44 gray 9780470
45 Lt Blue 9775187

Housings
40-43 tooth 25512339

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