I remember a post a while back that showed a website which showed what tooth gears you need based on rear end gearing for the speedometer to show the proper speed. I tried to search but couldn't find it again so if someone would be nice enough to help me out and send me the link again I'd appreciate that very much. Thanks.

 

Here ya go:

http://www.700r4.com/speedoCalc/speedocalc.html

 

Here's one for the T5
http://www.hanlonmotorsports.com/T5Speedo.asp

Pete

 

Yeah, i checked those out, some miss a few gear options, just give me your current specs and I will calculate the real most accurate gears. Just tell me what size tires, and rear gears you are running, and I will find you the correct gears.

 

can someone post the actual formula for the equasion? i've had a goofed up speedo for a year now, and witht he second tranny in with the speed gears not being correct i get a little nervous on the highway.

Tire size: 215-60 r 15
gear: 2.73 i think, unless i was never imformed of them being changed from the previous owner.

 

(Drive teeth X Axle ratio X Tire RPM) / 1001 = driven teeth

In your case:

Take your tire diameter: 25.16" and use it to find your tire RPM, which is (20168) / (diameter).

and calculate your tire RPM: (20168) / (25.16) = 801.59

Now, for drive teeth, you have several different options: 15, 17, 18, 19, and 20. The driven teeth are: 18, 19, 20, 21, 22, 23, 24, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45.

You need to calculate the closest one to perfect, which results in guess and check.

The most common drive gear is 15, so start with that.

(15 X 2.73 X 801.59) / 1001 = 32.79

According to this, your most accurate driven gear is a 33 tooth, which looking at the chart, does not exist, and a 34 would be a bit more off than you like, so try again with a different drive gear.

(17 X 2.73 X 801.59) / 1001 = 37.16

This is closer, and you can get a 37 tooth driven gear, so it is looking good, but you still have more options, lets see if we can get even closer.

(18 X 2.73 X 801.59) / 1001 = 39.35

That is worse, try again.

(19 X 2.73 X 801.59) / 1001 = 41.54

That is not very good, one more time.

(20 X 2.73 X 801.59) / 1001 = 43.72

Ok for a 44 tooth gear, but not great.


After all these calculations, it seems your most accurate gears will be a 17 tooth drive gear with a 37 tooth driven gear.

You will also need a new sleeve.

Take these numbers down, and order them form GM:

#8640517 - 17 tooth drive gear
#1359271 - 37 tooth driven gear
#25512340 - correct sleeve (if needed - you might already have the right one )

 

my tire size will either be 275/40 R 17 or 315/35 R 17 but I don't know where to find the diameter in inches.

my rear end ratio will be 3.42
Also, where can I find the part numbers for the different speedo gears so I can order the set that I need?

Thanks a lot, the help is greatly appreciated.

 

The 275/40/17 tires are actuallyt he same exact diameter as the popular stock 245/50/16, so if you had those 16" tires and the same gear ration, no change is necessary. The tire RPM is 784.747 so you can make with the claculations if you want, but I am pretty sure the 15 tooth drive gear is the best for this situation... lemme sut run the quick numbers, you can do the rest of the math yourself if you want to try other drive gears.

(15 X 3.42 X 784.747) / 1001 = 40.22

So 15 tooth drive gear with a 40 tooth driven gear is most likely your best bet, but feel free to run all numbers until you find somethig else you like.


Thise site has all part numbers you will need. Make note of the sleeve required, plus it has all the same formulas I use: http://www.73-87.com/garage/


After looking over his formulas I spotted this: "40300/1001 = 40.26 or 41 teeth"

I would have chosen 40 tooth gear as it is closer to the actual speed... why he chose 41 instead, beats me.

 

What about us guys with electric speedos?

 

Considering my car is a '90, which has the same speedo as yours, the gearing is the same, allthough I think the actual parts and part numbers are different. Do the same calculations with the drive and driven gear teeth, then go to GM and tell them you need those gears for an electric speedometer. It wasn't much of a problem for me, I told them I wanted and they got it for me... they took their sweet time getting them to me, but they got there nonetheless.

 

Thanks a bunch. The main reason I was asking is because I'm swapping in a rear with 3.42's as opposed to the 3.08's in there now so I wanted the speedo to be right. I'll go down to the dealer and let them know what gears I need for everything to be right. Thanks again.