Automotive Speedometer Gear Calculator Version v1.1
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.
Instructions for using the Speedometer Gear Calculator: Enter the following data -
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
Gear Calculator
or
Tire Size Calculator
to find this
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!
Speedometer Gear Calculator
Drive Gear Teeth
Rear End Ratio
Tire Height
Speedo Revs/Mile
Speedometer Gear
Speedometer Gear Teeth
Another simple tool from
www.gtsparkplugs.com