VCCA Home Forums TECHNICAL - GENERAL Mechanical - Engine, Trans, etc speedometer

if my speedometer is reading 50 mph at 45mph and I have 39 teeth on my speedometer gear on my TH400 tranny how many teeth should I go up to compensate the 5 mph difference.Thank You,Tony

GM products go by color of the speedo gear. Pink has 7 teeth, Black has 8 teeth and is 1.79" in diam., Orange has 8 teeth and is 1.84" in diam., Green has 9 teeth and Purple has 10 teeth.

You can see what color you currently have and then replace it with a color that brings you closer to an accurate reading. I'm sure there is info. on the web
that will tell you what the difference is in mph between the colors.

They're pretty inexpensive to buy, so I just buy one of each color to have on hand.

Bobalouie

You should really go by what the odometer says, not miles per hour. That is the way it is designed, and also you would then know if is a problem with the gear or the speedometer itself. IMHO check the odometer against some mileposts and see how many percent off it is.

Ok lets say you find it is an 11 percent error. 5mph high at 45mph is about an 11 percent error so lets go with that for now. 39 x 1.11 = 43.29 (111%) so about 43 teeth.

Here's a quick link that will actually walk you through the process, explaining how to calculate mph increases or decreases, and how that translates into the correct speedo gear(s).



http://chevellestuff.net/tech/speedo.htm

I agree with bloo’s recommendation to check the odometer accuracy before making any changes.

Some manufacturers calibrate the speedometer to show a higher speed than actual while maintaining odometer accuracy. Their philosophy is to make the owner think the car is faster than it is. Plus it might save a speeding ticket.

Bloo and Rusty give good advice!
Keep in mind that the odometer is a gear set moved by the cable. If the miles shown is correct, then the gearing is correct.
The speedometer is not directly driven. It is a magnet rotating inside an aluminum cup that is attached to the needle, the faster the magnet spins the more the cup and needle rotate against a spring.
A number of things can happen to cause the speedometer to be off. For example, the hair spring that returns the needle to zero can be weak or tangled. The delicate bearings for the cup and needle assembly can be gummy. The magnet may have lost some magnetism.
The bottom line is that if the odometer reading is not correct, then change the gearing. If the odometer is correct, but the speedometer is off, then replace or repair the speedometer.

Tire size will have an effect on speedometer accuracy also.

Also size relative circumference (diameter) of ring gear and pinion. Yes size. Number of teeth like on bicycle sprockets. doesn't matter. .

Best,

Charlie

This is a great calculator, MPH, Rpm, Tire diameter to use for most any variations! Link:
http://wallaceracing.com/calc-gear-tire-rpm-mph.php

My 34 Chevrolet Roadster new build will cruise all day long on the highway at 75 MPH with my motor RPM 2,400 ! Yeah that’s with a 4 speed overdrive with 485 HP SBC Stroker motor!

Oh I just think Charlie is a bit over in the engineering technology department with bicycles! Hahaha LoL ????

RoarsRods