1 DwnCam

I have a 92 camaro with 3.73's and 18" rims with 35 series tires. I would like to Re-calibrate my speedo but I dont know what speed sensor gear number I need, I have a 39 now. Also does anybody know the formula to figure it out?

Thanks.

Swapmaster

Here is a list of GM part numbers for drive and driven speedometer gears and some combinations that are known to work with the stock 26" tires.

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.

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
2.73 26.0 37
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
3.73 25.7 43

Omar

Figuring out what speedo gears your car needs:

You first need to figure out the height of your tires. The height of a tire consists of the rim and 2 sidewalls, top and bottom. Let’s say you have:

245/50/16 tires

We first do the height of one sidewall:

245 * 50 / 2540 = 4.82 inches (Dividing by 2540 converts from MM to inches)

4.82 * 2 = 9.64 inches (Multiply by 2 for top and bottom sidewalls)

9.64 + 16 = 25.64 inches (We are adding the height of the rim itself)

Since I did some quick rounding off, the number is actually 25.65 if you use a calculator and let it do all the work for you.

So we now have the height of 25.65 inches of our example tire.

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We now figure out what gears we need. There is a Drive gear and Driven gear.

Driven Gear = (20.168 * Gear Ratio * Drive Gear) / Tire Diameter.
(The 20.168 is an industry standard)

If you are using a 700R4, 3 drive gears are available:

15T (8642620) Gray
16T (8640517) Blue
17T (8640518) Red

We will use the 700R4 for our example. If you look in the tech section, you will see most popular combinations so you can determine which one of the 3 drive gears you already have. If you don’t know, it’s ok. It’s probably better that way, so you can replace that old worn out drive gear. So we will plug in all 3 drive gears and see what we come up with.

Let’s use our 245/50/16 tire with new 3.42 gears.

Using the 15T Drive gear:

20.168 * 3.42 * 15 / 25.65 = 40.34, or 41 driven gear

Using the 16T Drive gear:

20.168 * 3.42 * 16 / 25.65 = 43.03, or 44 driven gear

Using the 17T Drive gear:

20.168 * 3.42 * 17 / 25.65 = 45.71, or 46 driven gear

So our choices are:

15T drive gear, 41T Driven gear
16T drive gear, 44T Driven gear
17T drive gear, 46T Driven gear

Notice that when I did these equations, I rounded the answer up. This gives you some breathing room with the accuracy of your speedo. In the last case of 45.71, it probably would have been wiser to round up to 47. If the original answer was 45.99, then you definitely would want to choose 47. If you always round up, you will be safe.

Now we look to see what GM has available for Driven gears for the 700R4:

If you have a cable driven speedometer, use the following part numbers.

9774413 34 Lt Green
9780628 35 Orange
1359270 36 White
1359271 37 Red
1359272 38 Blue
1359273 39 Brown
1362048 40 Black
1362195 41 Yellow
1362049 42 Green
1362196 43 Purple
9780470 44 Dk Gray
9775187 45 Lt Blue


If you have an electric speedometer, use the following part numbers.

25513042 35 Orange
25513043 36 White
25513044 37 Red
25513045 38 Blue
25513046 39 Brown
25513047 40 Black
25513048 41 Yello
25513049 42 Green
25513050 43 Purple
25513051 44 Dk Gray
25513052 45 Lt Blue

Notice that there is no 47 driven gear to use for our 17T & 47T combo, so we can throw that one out the door. Using an average tire height of 25” or 26”, 3.73 gears is about the limit for correcting the speedo, unless you use a taller tire.

So we can choose:

15T & 41T

OR

16T & 44T

It also seems that the 16T drive gear wasn’t used with any combos, but it was available from GM. If you have a choice, go with 15T or 17T drive gears. The 15T seems to have more flexibility in case you want to change gears again in the future. Keep in mind that this is the harder one to change. So in our example, the 15T & 41T is the final answer.

Another thing to keep in mind is availability. It seems that GM has discontinued certain gears. This seems to be more prevalent with the T5 gears.

So far, I have seen these choices for the T5:

Drive gears:

6T
7T
8T
9T

Driven gears:

18T
19T
20T
21T
22T

FYI, this equation is if you know what driven gear you have:
Drive Gear = (.0495835 * Tire Diameter * Driven Gear) / Gear Ratio

If you happen to have a transmission other than a 700R4 or T5, simply do a search for what available speedometer gears were offered for your particular application. Then plug them into the above equations.

johndo1707

where would you get these gears?

Swapmaster

Your local Chevy dealer

jmd

iTrader: (4)

I can get them. I'll sell at the "middle" price on gmpartsdirect, plus actual shipping cost.