Hydromatic transmission Gear

Speedometer Gear Calculator


A simple to use Transmission Speedometer Gear Calculator. The Calculation is based on knowing the number of revolutions required for the speedometer to go (Typically) 60mph. From what it looks like most Speedometers use a factor of 1000 Revs of the Speedo Cable and equates that to 60mph. Your mileage may vary. I think most electronic speedo's work that way as well. Don't know how this maps to you metric folks, but if you know send me an email and I'll fix it up!

The images are various images of  speedometer gears that I found on the web. One is a nice cut away of a transmission
showing how they work in a GM transmission.
Speedometer Gear Calculator
Drive Gear Teeth
Rear End Ratio
Tire Height
Speedo Revs/Mile
Speedometer Gear
Speedometer Gear Teeth

Speedometer Gear Calculator Explanation


This simple calculator does the math to help figure out the 2 gears that are needed to make your speedometer work correctly. Many speedometers have a given RPM they must turn at 60mph to be correct. This calculator allows you to find a close gear set that will make it all happen. What you will need to find is the magic number that your speedometer really needs to turn. From poking around the web it seems many modern speedo's run 1000 RPM and display 60 MPH. For electronic speedometers this seems to be the common case. Other values are said to be stamped on the back of the Speedometer itself. One other thing to watch out for is the size of your tire. The Diameter of most (Yes even GTSparkpulugs) Calculators give the size of the tire unloaded. So if you have a 'Computed' Diameter of 25.75" for a tire, the loaded rolling diameter might be less, and so best method to get the number is to MEASURE IT! The output from the calculator is a floating point number which is done to give you a feeling of whether to round up (More teeth) or round down (Less teeth). So if your at 7.9 teeth, use a gear of 8 teeth. Also some modern electronic speedometers allow a PULSE count to be set. This adjustment can allow you to set the speed correctly without a gear change.

Instructions for Use


Drive Gear Teeth - Number of teeth on the transmission side of things
Rear End Ratio - Rear Gear Ratio (3.89, 3.08, 4.11, etc.)
Tire Height - In Inches, Use The Gear Calculator or Tire Size Calculator
Speedo Revs/Mile - Number of RPM the speedometer needs to turn at 60mph

You can hit the SAMPLE button to see how some typical calculations will look. RESET will clear out the form and the CALCULATE button will do the work. If you find any problems let me know!
Green Speedometer Gear
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Transmission Tailshaft speedo Gear
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