When you check your truck odometer for a replacement, note the gear taper that turns the five numbered mileage wheels. Some have straight gears and others tapered. Each have 15 teeth.
The rule on Chevrolet, GMC and other GM vehicles is as follows:
If the odometer gear teeth are straight cut, the unit fits below the speedometer needle shaft.
If the odometer gear teeth have a tapered cut, the odometer assembly fits above the speedometer needle shaft. The different teeth “cannot” be interchanged.
Here are a few examples: