[fireprix]

Unsure where to post this but I was wondering what to do to calibrate my speedometer. It is a turbo 400 trans in a 75 G.P. The rear gears were changed from 2.56 to 3.23. I know I need to replace at least 1 of the speedometer gears but would I need to replace both the drive and driven gears? If I need to replace both, do I need to replace the governor? For now the car has 225/70R15 tires. What is involved in replacing them? Where would be the best place to get the needed parts? Can this be done without pulling the trans? Any help would be greatly appreciated.

[74Grandville]

any idea what color gears you have now? it's hard to give you an answer with out knowing the Drive gear.

[fireprix]

No I haven't attempted to disassemble anything as of yet. As far as what I have heard, there are numbers stamped in the governor that specify how many teeth the easily replaced gear can have and it's usually only a range of approximately 3 teeth before you have to change the more difficult gear. The transmission is the original one to the car and it left the factory with 2.56 gears. Just would like to have on hand what I need before I take it apart if at all possible. Thanks.

[74Grandville]

if you at least uncrew the speeo cable and tell me the color of the driven gear i may be able to give some direction, but the color of the drive gear would be best. that would require pulling the speedo housing and looking inside.

[400 Lemans]

I have a chart but cant get picture to post but it calls for a 45 driven gear with an 18 drive gear and 70 series tires and 3.23 gears. You will have to change your aluminum gear housing to the 40 to 45 one. The one you have now should be for 34 to 39 driven gears.

[74Grandville]

Are you sure it has a 2.56 and not a 2.93? do you know what the original tire size was?G78-15, GR70-15

http://midsizebowties.net/pontiac-gr...ecs-1973-1977/

[JLMounce]

On the recommendation of another member, who I can't recall right now, I went with a California Performance Transmission external calibrator. All you do is get out your phone with a GPS app and make note of how fast your speedometer is reading when the GPS reads 50mph. Call up CPT, give them that info and they'll build you a calibration unit that attaches to the speedometer housing in-line of the cable and corrects the speedometer.

It's $75.00 plus shipping and installs in about 5 minutes.

[AJD1964]

I recently installed a 3.23 in the '71 Lemans, and with using 245 60 15 tires, a green 15 tooth drive (on trans shaft) & a red 37 tooth driven (in gear housing) speedometer gear set did the trick for me.I did not have to change out the 34 to 39 range gear housing I had already. I did have to remove the tail-shaft housing to install the 15 tooth gear and clip on the tail-shaft. It's not that hard to do, at least on a TH350. Being that you have a TH 400 there may some gear housing differences, but not 100% sure.

I'm within 2-4 mph as compared to GPS depending upon speed, as error increases with speed. With 3.23's I don't take it up to as much as 80 mph tops anyway. Go on-line and find a speedometer gear calculator, there are many out there that do all the math for you and makes it easy. The 3-piece set I bought was 30 bucks shipped. Easy to find on-line with many combinations.

[stevep]

What % are you off?

[fireprix]

Thanks for all the responses. It is usually hard for me to get anything done due to a demanding job. It seems to read about 15 MPH too fast at 55-60 MPH clocked with GPS APP but I would have to recheck since I'm going off memory. The correction box sounds intriguing but I'm building a mild 455 for the car and will probably have to put some larger tires and or wheels on it down the road. Would be thinking 17" Rally IIs or at least some 60 series tires on the 15" Rally IIs so I'd like to not have to change the difficult gear more than once if avoidable. Thanks again.

[pastry_chef]

Just a guess, maybe 18 drive 34 driven now.

If this is true going 43 driven would be darn close.

[Goatracer1]

If you are off more than 5 percent you will have to change both gears. The easiest way to find the correct speed compared to the speed on the speedo is to use a GPS that also reads out your speed.