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:
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.