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If you are changing your tire size or rear axle ratio in your vehicle, this change will make your speedometer inaccurate. You will either need a MECHANICAL ADAPTER for an all cable design or the ELECTRONIC RATIO ADAPTER (ERA) for all electronic design. For customers with a mechanical speedo in a vehicle equipped with an electronic speedo signal, we offer the Cable X Box.

Mechanical Ratio Adapter – $150.00 + shipping
Electronic Ratio Adapter – $155.00 + shipping

Mechanical Ratio Adapter- In most vehicles, the speedometer calibration is based on a certain tire and transmission ratio. These factors were used by the speedometer manufacturer to establish the proper combination of gears and worms inside the speedometer head to obtain accurate speed readings. Some car manufacturers and individuals building customized cars are using different tires and/or transmission ratios rather than OEM equipped. If this is the case, the ratio has to be established. In general, if the tires or transmission, including differential of your car has been or will be altered by you, or if any other changes have been made that are not listed as original or factory approved equipment, the ratio has to be adjusted. The following tests will tell you if the error you are experiencing is in the speedometer gauge itself or is caused by cable delivery from the transmission. After performing the test, call us at 1-800-592-9673 or 1-810-632-0400 with the results.

THE EASIEST WAY TO TEST RATIO: Stop at a mile marker found on most US highways. Record the odometer reading, including the 10ths. Drive 10 miles and stop at the mile marker. Record the odometer reading again, including the 10ths. Subtract the first reading from the last reading. This will tell us if the cable delivery (ratio) is correct or incorrect. If correct, your subtraction answer will be 10.0. Conclusion: If odometer reads correctly per 10 miles driven, the error you are experiencing is in the speedometer head. If reading is incorrect, the error is from your transmission and the ratio must be corrected. Although rare, you could have both cable delivery and speedo head problems. In any event, your cable delivery must be correct in order for the speedometer to be accurate.

FORMULA FOR FIXING AMERICAN MADE CARS: Generally, American made vehicles with mechanical speedometers need 1000 cable revolutions per mile to make the odometer record correctly and the MPH to be accurate. Here’s a simple formula for you to fix many American made car ratio problems. Conduct the 10 mile test, remove driven gear (the gear that spins your cable), count the teeth, and then multiply the teeth times the subtraction answer from the test. Call us at 1-800-592-9673 or 1-810-632-0400 and we will supply you with the correct gear or we will build a ratio adapter to fix the problem.

Electronic Ratio Adapter- The automotive industry has a great variety of part sizes, types, shapes and descriptions, but regardless of make, model or year, a few things are standard. One of those standards has been the number of revolutions a speedometer cable will make for each mile travelled. Nowadays, electronic pulses have replaced the rotating cable, but the same principle applies. The Pulse Ratio (the number of pulses per mile travelled) remains the same, regardless of speed, since the same distance is travelled and the same number of pulses have occurred each mile no matter what the speed was during that mile. However, this Pulse Ratio can be made to vary from the true when modifications are made which change the number of electronic pulses per mile on a particular vehicle. This is most commonly caused by changing the tire size (increasing the outside diameter of the tires will cause the tire to travel further before making a complete revolution), but other modifications could have the same result. When the Pulse Ratio is thrown off, the speedometer/odometer will be inaccurate and corrections must be made. This relative difference between true speed and the speed indicated on the speedometer is called the Variance Ratio, and it is corrected using an Electronic Ratio Adapter. It’s a snap to install: input, output, hot, ground. Set the dip switches followng the Calibration Table on the installation instructions enclosed with your ERA and you’re ready to go.