I remember seeing a post with a formula for figuring out the amount of teeth needed on the speedometer gears by plugging in tire size and rear end ratio. Does anyone have the formula, or know where that post is? I've tried searching and can't find it. Thanks.

 

Q: How to I calibrate my speedometer for new rear-end gears?

A: Determining the precise speedometer drive gear/driven gear combination required for a rear axle ratio change is surprisingly easy-if you know the existing combination and axle ratio. Whip out your calculator and follow along. Let's assume that your F-Body is currently equipped with 2.73 gears and that you're planning to install a 3.73 ring and pinion. If you didn't change speedometer gears, actual vehicle speed would 73.1 percent of indicated speed (2.73/3.73 = .731). That being the case, what's required is speedometer gearing that will result in the driven gear spinning at 73.1 percent of its current speed. The speedo gear combination for a 2.73 rear (assuming 26-inch diameter tires) is a 17-tooth drive gear and 37-tooth driven gear. Switching to a 45-tooth driven gear (the most teeth available) would translate to the vehicle traveling at 82 percent of indicated speed - still a 10 percent error.

Obviously, the drive gear must be changed to one with 15 teeth if the 73.1 percent ratio is to be achieved. Going back to the original 37-tooth driven gear for illustration purposes, the 15-tooth drive gear would result in the vehicle traveling at 88 percent of indicated speed - that seems like a step in the wrong direction, until the driven gear is changed. (This is where it gets a little tricky because you have to work with a percentage of a percentage). Matching the 15-tooth drive speedometer gear up with a 44-tooth driven gear brings the drive ratio to 74 percent (15/17=.88; 37/44=.84; .88 x .84 = .739, which rounds to .74, or 74 percent). Still not quite enough. Will a 45-tooth driven gear do the job? Run the numbers through your calculator and you should come up with 72 percent. That's about as close as you're going to get.

In case you haven't put all the pieces together, there is a distinct limitation regarding speedometer accuracy and steep rear gearing. Unless driven gears with more than 45 teeth, and/or drive gears with fewer than 15 teeth become available, it will be impossible to have an accurate speedometer with rearend gear ratios lower (higher numerically) than 3.75:1 - unless 27-inch or larger tires are installed.


Q: I want to buy speedometer drive/driven gears, what are the part numbers and combinations?

A: Here is a list of GM part numbers for drive and driven speedometer gears, as well as some combinations that are known to work:



Part No. Teeth Color Sleeve*
25513042 35 Orange 25007338
25513043 36 White 25007338
25513044 37 Red 25007338
25513045 38 Blue 25007338
25513046 39 Brown 25007338
25513047 40 Black 25007339
25513048 41 Yello 25007339
25513049 42 Green 25007339
25513050 43 Purple 25007339
25513051 44 Dk Gray 25007339
25513052 45 Lt Blue 25007339

*Some applications may use Sleeve No. 25007224 or 10456092 in place of 25007338, or 25007308 or 1045089 in place of 25007339.
Red Drive Gear Combinations

With drive gear number 8640517, (red, 17 teeth) the following combinations apply:

Axle Tire Dia. Driven Gear
Ratio (Nominal) (Tooth Count)
2.59 26.0 35
25.7 36
2.73 26.0 37
25.7 38
2.87 26.0 39
3.07 26.0 42
3.31 26.0 45

Gray Drive Gear Combinations
With drive gear number 8642620, (gray, 15 teeth) the following combinations apply:

Axle Tire Dia. Driven Gear
Ratio (Nominal) (Tooth Count)
3.33 25.7 40
3.45 25.7 41
3.54 26.0 42
25.7 43

Lower ratio (higher numerically) rear or smaller diameter tire requires higher driven gear tooth count. Increasing or decreasing driven gear tooth count by one will usually accommodate a 1/4" to 1/2" change in tire diameter.

 

hear's on for you, what about manual trannys, they use diffrent gears? (I just went to the dealership and they gave me a blue drive and purple driven and it worked, but what about diffrent ratios)

 

Well, I have a 25.5 inch diameter tire and a 3.42 rear end. Does this mean a gray 15 tooth drive gear with a yellow 41 tooth driven gear would be the closest? Thanks.