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Old 04-03-2005, 08:47 AM
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Speedometer Information.

This thread is meant to answer as many questions as to:
How do I correct my speedometer?
Why does my speedometer needle wobble?
I just changed my rear-end gears why is my speedometer so far off?
What gears do I need to correct my speedometer?

(Q) I got an instrument cluster from a wrecking yard. Can I use it in my project vehicle? Can it be reset to the mileage I want?
(A) It is a felony in most states to tamper with odometer readings, honest people in the business or repairing such mechanisms are NOT willing to risk it for the sake of providing you with a counterfeit "low mileage" car -- no matter how nice you are.

This has been a project that I wanted to work on for quite a while, I was going to hold off until I got things worked out the way I wanted, and to make it as easy as possible, BUT here we are with little to go on.

However, you are allowed to repair the speedometer mechanicals to allow the reading to be as correct as possible, the ODOMETER is the "rolling" number counter that tells the vehicle operator how many miles the vehicle has been driven, the odometer is a PART of the speedometer.

The speedometer is a mechanical calculator comprised of a drive gear, a driven gear, at least one cable and the speedometer head. The drive and driven gears can be changed to allow correct readings in tire and axle gear changes, but to a certain extent without an additional gear ratio adapter.

Newer computer controlled vehicles speedometers are electronic units that use specific parameters, signals, and calculations for that specific vehicle to show the correct speed, I WILL NOT COVER THESE AT THIS TIME- UNLESS SOMEONE WANTS TO HELP.

Mathematical equations related to gearing:

Ratio = (rpm x tire diameter) divided by (mph x 336)
RPM = (mph x gear ratio x 336) divided by (tire diameter)
MPH = (rpm x tire diameter) divided by (gear ratio x 336)
Tire Diameter = (mph x gear ratio x 336) divided by (rpm)

Online calculators:

This one tells you the size and MPH differences when switching tire sizes.
Corvetteactioncenter.com's Speedo Calibrator

I have used this link frequently in the past.
This is a tire size comparison calculator.

This link does the math for finding out MPH with many parameters you can change.
A MPH Calculator from RPM and Gearing

This link gives some ratios with referenced tire sizes, and part numbers for correcting speedometers in a Jaguar/Chevy V-8 swap.
Chevy TH-350/400/700R4 in a Jaguar.

A freeware RPM calculation program I found this morning.

This tells what gears are needed for correct speedometer readings, but DOES NOT cover all transmissions.
Bowtie Overdrive's Speedo Calculator.

This link also lists gears available.
TCI's Calculator, this requires you to already know what drive gear you have.

This link shows available speedo gears, gearing reducers and recommended combinations for correct readings.
Speedometer parts and calculations.

Try the search link above (it has a small dropdown arrow immediately next to it and search for what you need through the 14 pages of results. Just type in "speedometer". This subject has most likely had your particular question covered in the past. I will eventually have links to the most important and relevant information for almost anyone to find what they need.

I will have A LOT OF INFORMATION when I get this done, however long that may be.
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Last edited by M&M CUSTOM; 07-15-2005 at 06:15 PM. Reason: Just updating, again.
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