TH400 speedo calibration-plastic gears?

I now one of you smart guy's here can tell me some web site that has a chart or a way to figure out what color plastic gears you need for a given gear ratio/tire size to correct the speedometer calibration.

I know GM had to have had a chart or something to let them know that a car with a 3.55 gear and a G70-14 tire used a purple(or whatever color) plastic gear set in the trans to make the speedometer correct.

Seems like there would be,or would have been a chart for dealers or mechanics to tell them what color plastic gears to install if they did a gear ratio change.

Does something like this exist, or am I dreaming?

Thanks,
Tom

I have a guy that want's me to do a gear change and posi install on his car, but he want's the speedo to be correct when I'm done.

 

If you can't find the info here.....

http://www.transmissioncenter.net/speedometer_calibration_______va.htm

you're not likely to find it anywhere. :Comp:

And the speedo gear calculator is here.....

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

 

AND-the rule of thumb, which I have found to be very accurate, is 2.5 mph per tooth. more teeth=slower speedo. You likely know the mounting sleeves have to be moved around, right? The sleeve will have which gear tooth size it will accomodate on the end-36-37-38-39 is the one I have in my hand. and sometimes you cannot get there with the available gears and have to use an angle speedo adaptor-I got the 3.89 gears with the 30.5 inch tall drag radials spot on like that in old orange using my gps for accurate readings to dial it in.

 

I think this chart is for a 4 speed, I dunno if they are the same for an auto..........

 

There are charts that list which driven gear to use with what gear and tire combo, but you will also need to know which drive gear is in the transmission output shaft.

 

HERE is a chart from a Pontiac parts book.

I don't know of any others online.

 

You can experiment with gear combinations with TCI's calculator as well.

http://www.tciauto.com/Products/TechInfo/speedo_gears.asp

Devon

 

Thanks Guy's:TU: I knew I could count on you all.

 

Someone should make a sticky out of this formula. It is a simple matter of multiplication and division.

Mechanical spedometers are calibrated to read 60 MPH @ 1001 RPM.

First...every dimension should be converted to match. Inches is easier than miles...so...

63360 = inches in one mile.

63360 / pi / tire diameter in inches = tire revolutions per mile
Tire RPM X Final drive ratio = Engine RPM @ 60 MPH
Engine RPM @ 60 MPH / 1001 = Multiplication factor
Multiplication factor X number of teeth on speedo DRIVE gear = number of teeth on speedo DRIVEN gear

Now your calculator can substitute for a web site.

 

Then why not use an equation relying on tire radius from center to ground? I know, I know. Splitting hairs.

Devon

 

Radius could be used, as could circumference. But most tire manufacturers spec their tires by diameter (and width), so I use that dimension. Some manufacturers will list revolutions per mile of their tires, thus simplifying the formula.

And whats wrong with splitting hairs? I do it all the time:grin:

Jim.

 

I mentioned -Z radius because most of us forget how much a tire "flattens" which affects the formula. But then you'll ask me about radial increase vs surface velocity and I'll have to sign off and go for more coffee for not having any clue as to what that might be.

Devon

 

If you want to get into spherical triginometry we can do that too, but I'd rather get another cup of coffee. It is far less complicated.

Jim