Speedo Calibration
Short answer: There are three ways to recalibrate, and they work well together.
- Measure & compute. Find how far off you are (e.g., read the odometer over a 10-mile measured stretch — say it reads 1.4× high). Pull the speedo gear/housing from the trans and count teeth on the driven gear and the drive gear it meshes with. New ratio needed = (driven teeth ÷ drive teeth) × your error factor. Driven gears only come ~35–45 teeth, and a big change also needs a different housing (each houses a range, e.g. 36–39, 40–43), so aim to stay within your current housing range (housings ~$25, gears ~$7).
- Formula (use as a sanity check):
Drive Gear = (.0495835 × Tire Dia × Driven Gear) ÷ Gear Ratio
Driven Gear = (20.168 × Gear Ratio × Drive Gear) ÷ Tire Dia - Call a specialist for GM part numbers — Ronnie Redd at Jerry Brown Chevrolet, Buford, GA, 770-945-4981 (have your gear ratio, tire size, and trans type ready).
| Teeth / Color | GM Part # |
|---|---|
| Drive gear, 17 tooth | 1246221 |
| Driven 36 (white) | 1359270 |
| Driven 37 (red) | 1359271 |
| Driven 38 (blue) | 1359272 |
| Driven 39 (brown) | 1359273 |
| Driven 40 (black) | 1342048 |
| Driven 41 (yellow) | 1362195 |
| Driven 42 (green) | 1362049 |
| Driven 43 (purple) | 1362196 |
| Driven 44 (gray) | 9780470 |
| Driven 45 (light blue) | 9775187 |
| Housing, 40–43 tooth | 25512339 |
Any proposed updates, changes, pictures, and/or corrections, please use our comment section below (may need to click on permalink to access comments feature). Information is subject to change and offered as is without any warranties or guarantees. Please review our Term's Of Use for more information.
Content last modified: June 13, 2026 at 10:06 am
← Frequently Asked Questions
Taking this one step further….Recently purchased ’68 Firebird and noticed speedometer was LOW 20% at 50 MPH and odometer was HIGH 20%. Thought a fix of the speedometer by 20% would fix both but…..Had 38 tooth driven gear in it and replaced it with 34 tooth driven gear. Speedometer is now within 2 MPH at all speeds up to 60 so speedometer is fixed as much as it can be (there are no options for less than 34 teeth driven gear). However, odometer is now 30% HIGH (reads more miles than traveled). Have checked both with Iphone app that I know is accurate from other cars. Have zero idea how a speedometer can be correct (or close enough) and an odometer to be even further off than it was with the 38 tooth driven gear.
What could cause this other than faulty odometer internal gearing.
Anyone have another idea?