Posted this on the tranny board and could not get a stright answer..............Ok I am changing gears in my formula going from 2.73's to 3.42's....Can anyone tell me What Drive and driven gears I need.....BTW I have a 700R4 Tranny and 25.5" tires..........Thanks for the help

 

Umm, plastic ones?

Sorry, I can't be of much help on that one, but at least it will hit the top of the page again.

 

here is a # 760-947-5240 they can tell you for sure the name of the place is bow tie overdrives they also build the strongest 700r4 tranny i have ever owned I have a low 11 sec 70 camaro street car with one of there trannys and it has been in there a year and a half no other tranny would hold up more then 6 months. I also have 342 rear gears and 295 50 street tires but i forgot what color my gears are i think maybe purple on the shaft and grey on the speedo but not sure. hope that helps you. they are in cali you can visit them at
www.bowtieoverdrives.com
email dr700r4@earthlink.net
peace and good luck

 

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

 

Omar,

I don't believe I have ever seen such an extensivley detailed reply such as that in all my life!! Hopefully now I'll be able to figure out what the hell I have for rear gears in my car.

Also, do you have any pics of your brakes with the wheels mounted to the car?

Robert

 

Ummmmmmm........ ..ok ...........I love that answer but...............could you just come right out and tell me what I need.........I do have 245/50/16 tires........not real good at all this math stuff ......... :lala: :lala: ....I would be forever in your debt...........but I will say this Omar...You are the man.............:hail:

 

Omar,
Great reply dude. You saved me a lot of time. I just changed my driven gear and counted the teeth on the old one. It turns out that the drive gear was a 17 tooth, and all I needed was to change the driven. Thanks man.:hail:

idrag2:
From my calculations you need a driven gear with 46 teeth. The FAQ section suggests that they dont make one that big, 45 is the biggest. You may have to change you drive gear, or just settle for the 45. Its pretty close. I've got a brand new one if you need one (the dealer orriginally gave me the wrong one)


Chris

 

Chris,

How much for the gear?....................And have any of you guys heard of Southern Gear.......if so are they any good?


P.S. How many teeth do I need on the drive gear?