Hi, I just put in a 4th gen rear size 3.23, I used to have a 2.73 with 16 245/50 tires now I have 245/45/17 tires with the 3.23 rear. I've tried the tech section but I'm not good at calculations. Is there an easy way on how to calculate what speedo and drive gears I need. Oh, I do have the 700 tranny. Thanks.

 

15T Gray (8642640) Drive, 39T Brown (25513046) Driven
or
17T Red (8640517) Drive, 44T Dr. Gray (25513051) Driven

 

Hi Aperion, Thanks for the reply but how did you calculate these gears. Or if any other person can help. I like to be double sure before I purchase any gears.

 

Check out www.f-body.org. They have a link that can give you the info you need.

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This is from one of my older posts:

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.

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, 16T, 17T

If you are using a T5, I believe there are 4 drive gears: 6T, 7T, 8T, 9T
(Someone can correct me if I am wrong about the T5)

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:

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

Keep in mind that these part numbers are for cable driven speedos. If you have an electronic speedo, use the part numbers in the tech article. The teeth are the same (34-45) but the part numbers are different.

So we can choose:
15T & 41T
OR
16T & 44T

But 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.

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.

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:
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 anyone wants to add to this, such as the T5 info, or correct me, PLEASE feel free to jump in. I have pretty much gathered this information from studying different web sites, and I am in NO means an expert in this field. There maybe some different issues with VSS, electronic speedos, etc. that someone could enlighten me on.

You may also need to change your speedo sleeve, if the gearing change was drastic enough.

GM tech has a good article on the actual changing of the gears:

http://sethirdgen.org/vss.htm




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1989 IROC-Z 5.7L
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