Following is a tech. question and answer about speedometer accuracy. 

Dear Tech guy,

I changed the rear axle gear in my Pontiac from a 3.08 to a 3.42. The car has a T-400. It seems to run better, but the speedometer reads about 6 MPH fast. What is the easiest way to fix it?

The speedometer accuracy is determined by the gear ratio between the speedometer drive gear and the speedometer driven gear (both in the transmission). The driven gear is accessible after removing the speedometer cable (oil will not leak out with this step), and removing the Sleeve & Seal Assembly that is held in by a Y retainer and a 1/2" bolt. NOTE: A quart or more of oi1 will run out when the sleeve assembly is removed.

Clean the area around the sleeve prior to removal, catch the oil in a clean container, and pour the oil back in when finished.

When a speedometer is in error for any reason, (gear change, tire size change, or factory error), determine the amount of error and convert that to a percentage. Example; The speedometer reads 66 MPH at an actual 60 MPH. 66 divided by 60 is 110% or 1.1. That means the driven year is rotating 1.1 times too fast. To slow it down, we must increase the number of teeth on it by 1.1. If it is a 40 tooth gear initially, a 44 tooth gear will be correct.

Conversely, if the speedometer is reading too slow, the driven gear speed must be increased, and less teeth on the gear would be required. In John's case, the speedometer was accurate before the gear change and the error caused by the gear change is known. 3.42 divided by 3.08 = 1.11. John needs to increase the driven gear's teeth by 1.11. If he has a 40 tooth gear, 40 x 1.11 = 4.44. Therefore, he would need a 44 tooth gear.

As the speedometer and the odometer accuracy relationship are determined by the speedometer assembly, it is unlikely that both will be totally accurate even though the correct gear is installed. Generally, it is best to have the speedometer as accurate as possible.

T-350 and T-400 Sleeve Assemblies are not interchangeable, and each transmission has two designs of Sleeves. One Sleeve will hold driven gears from 35 to 39 teeth, and the second Sleeve will hold gears from 40 to 45 teeth. You may have to change the Sleeve depending on the new required driven gear. Both gears and Sleeves are available through GM dealers. In order to utilize these 11 gears for every application from 2.21 geared Catalinas with large diameter tires to 4.11 geared "Birds" with small diameter tires, several different size drive gears and even adapters that fit between the cable and the transmission were used. Depending on the transmission Model and your tire/gear combination, a driven gear change may not be enough. In that case, call your friendly (local) Tech Guy.