Speedometer Gear Chart 200-4R

After changing rear end gears or tire diameter you will likely need to swap one or both speedometer gears to keep the speedometer reading correct. To determine which speedometer gears are needed, you need to know two things, tire diameter and rear end gear ratio. If you plug those numbers in below and click the Calculate button, it will calculate the the ratio the speedometer gears need to be for the speedometer to read correct:

 Rear end gear ratio (ex: 3.73 or 4.10) Tire diameter in inches:

Once you know the ratio you need, look through the chart below to see which combination of speedometer gears is the closest match. The chart is arranged by ratio from low to high.

The second row (in bold) is the stock gear combination for a 85-88 Monte Carlo SS with stock 3.73 rear gears and stock tires, which are approx 26" diameter. If you plug those numbers in the calculator, it says a .345 ratio is needed. However, I confirmed with several other 1985-88 Monte SS owners that the 10/30 speedo gears are stock. If you know why GM chose to use the 10/30 gears instead of the 10/29 combination, let me know!

 OE GM# Drive Gear Driven Gear OE GM# Ratio 8639906 10 31 403417 0.323 8639906 10 30 561035 0.333 8639906 10 29 403999 0.345 8634965 11 31 403417 0.355 8639906 10 28 416765 0.357 8634965 11 30 561035 0.367 8639906 10 27 410731 0.370 8634965 11 29 403999 0.379 8639906 10 26 25502669 0.385 8634935 12 31 403417 0.387 8634965 11 28 416765 0.393 8634935 12 30 561035 0.400 8634965 11 27 410731 0.407 8634935 12 29 403999 0.414 8634934 13 31 403417 0.419 8634965 11 26 25502669 0.423 8634935 12 28 416765 0.429 8634934 13 30 561035 0.433 8634935 12 27 410731 0.444 8634934 13 29 403999 0.448 8634935 12 26 25502669 0.462 8634934 13 28 416765 0.464 8634934 13 27 410731 0.481 8634934 13 26 25502669 0.500

Each .007 of ratio equals about 1 mph of error at 60 mph. If you choose with a lower ratio (numerically) pair of speedometer gears than required, the speedometer will read less than actual speed, and vice-versa.

Since the ratio you need will likely fall between two of the available gear combos, most people choose the higher ratio combo, so the speedometer reads a little faster than actual speed.

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