I realize Ill get flamed away for asking without all info but Ill try anyway...

Factory setup was 700R4 tranny, 9 BOLT REAR with 3.27 Ratio, and the 16" factory tire size gta Rims and tires.......

What Im switching to....

Same Tranny.....Same stock 16" rims with same stock tire size.....Only difference is I will now have a 3.70 rear(Ford 9") end ratio..... My thought is I can get it close enough if I get whatever gear is used for 3.73...

Sorry but I have no clue what color gear is on my tailshaft or on the vss....??? sorry to post this one but Im at a loss and figured I would try to get help before ripping everything apart to check internal gear color..

 

I'm currently working on a tech artical on the this subject. Your really going to need to find out how off the speedometer is with the new setup.

Example your speedo reads 60 and your really going 50, and you have a 38 tooth driven gear. You will need a 43 tooth driven gear to correct the speedometer. Every 1 tooth increase/decrease equals 2mph so in the exapmle you would need a 43 tooth driven gear to correct the problem but the housing the gear sits in needs to be changed to an offset housing/vss module because the gear will be bigger.
The standard housing vss or cable driven fits 36-39 tooth drive gears.

GM was useless on helping me out in my situation too much math involved for them so I had to figure out another way to work things out.

 

Well I could try that but here is what BLOWS...Im putting a rear end in....So all this crap will be disassembled then....Then I install everything.....Then paying a cop to shoot a gun at me I find out I have Ratio XYZ and need this gear.....Then I take everything back apart and put in new gear...My original idea was to try and work out something close so I could install everything at the same time...

HMM Maybe Ill just get off of my lazy *** and check what color gears I have...LOL...at least that would be a start...

Thanks for the info...Let me know when the article is done...

 

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, 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

 

One problem with this setup or at least the line I'm getting from MR. dealer. The offset housing for larger drive gear 40+ is not avalible any longer cable driven or VSS, which doesent make sence because the actual speedo housing for a 700r4 is the same for a TH400, B-O-P TH400 and a 700r4. Just in case any 87+ VSS guys were planning on a TH400 the vss unit will fit that tranny. The correct driven gear is avalible for a 10 bolt 4.11 rear end you have to ask for the gear for a TH400, just like the 170* thermostate that doesent exsist on the GM books.



SSC

 

OK guys thanks for the help that clears it up a lot...Looks like I do some quick match and figure out what I need...

 

There is a calculator at 700r4.com
Should the formula give a "spot-on" result or is it some safety factor involved?
I installed the correct gears, but measuring with GPS shows that I should have chosen a 40 teeth driven gear instead of the calculated 39.