Mark A Shields

I wouldn't really care except for the fact it messes with your milage. I forgot what all it's called, do they go by colors or something. What do I need to order? I thought they were like $30 or something.

Oh, yeah I got 3:73s.

SSC

The speedometer drive gears need to be changed.
You best bet is to call the dealer and tell them tire size, tranny type and rear ratio. They can usually find you what you need in stock unless you have to deal with a woman doesent have a clue. <-- lady that always answers the phone at the local dealer here, cant find me speedo gears for stock 14s, 3:42' and 700r4 it , Go figure. Im probably going to swing by there at lunch and talk to a guy, if you can get me your tire size I can probably get you a part#
SSC

gcore45

I got a drive and driven gear to fix my speedo for less than $30. The dealership wasn't able to do the math, so I went to the faq for gears on this site and did it myself. The math is a pain but if you do it in an excel spreadsheet you can figure all the gears to get the ratio right. My speed is now dead on.

Jay 727

I tried the GM dealer, waste of a trip. Then I checked tciauto.com, figured out the gears I needed and posted it on this board and got a reply with a part number. Went to the dealer again with a part #, they love that
You could post tire diameter to help others figure it out.

redbird_400

I tried Astro transmission and the dealership + some junk yards. None of them seem to have a clue so the next best thing is to search the web. Here is a little tool that should help you find what size spedo gear that you need. Don't give up get that thing fixed.


http://www.bgsoflex.com/speedo1.html


damn trans shop chargeing me $150.00 and they cant even do the job right.

ryn28

iTrader: (1)

I bought a ratio adapter. It was a simple unscrew and screw back on installation. I called and told them what tranny I had, my tire diameter, and what my rear gear was. Then they made me the adapter. It screws in between the speedo cable and the trans. It was slightly more money then the gears but the install couldn't be cleaner or easier.

http://www.speedometershop.com/ratio.html#ADAPT

http://www.speedometershop.com/

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.

----------------------------------

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, 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.

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.

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

Jay 727

Not to be a smart *** but is this for people who own a calculator and not a tape measure? The best way to measure a tires diameter is to put an 18" level accross the top and measure to the ground, a tires calculated diameter and loaded diameter can vary. Same sized tires from different manufacturers can vary.
I'll write that formula down though incase the guys at NASA ever post here.

Omar

Jay, you are right. A 245/50/16 tire made by Goodyear maybe be slightly taller or shorter than one made by BF Goodrich. But when you are talking about the accuracies of our speedo gears, that difference in height is not going to make a difference. Our speedo gears are not dead on accurate.

You suggested going to www.tciauto.com and using their online calculator. I noticed that there was no field to enter "tire manufacturer". So how is what I suggested any different than what you suggested?

I just showed people how the calculator works and what is computes, so they could do it themselves.

Not to be a smartass or anything.

Jay 727

I'm glad you didn't take that wrong and I'm not mad.
The way I said to do it measures the tires diameter as it's on the car. TCI's site asks for tire diameter not size as marked on the side wall. I figured it's not that critcal anyway because usually it's told me I need gears like 42.21 when the closest thing is a 42 anyway.
Thanks for all those part numbers:hail:

Apeiron

Or if you don't want to do all that dirty work, you can always try <a href="http://members.home.net/crussel/speedo.exe">this</a>

<a href="http://members.home.net/crussel/speedo.exe"><img src="http://members.home.net/crussel/speedo.gif"></a>

redbird_400

Can you use that for a electric speedo?

ryn28

iTrader: (1)

No, the ratio adapter like the one I listed would not work. For electronic speedometers you'd need one of these electronic units:

http://www.specmo.com/New_Files/Cali...120%20Overview

It's more expensive, more then twice the cost. The good thing is it looks like you can change the ratio of the adpater with dip switches. If you change gears or tire sizes more then once it would be simple to set the speedo calibration again.