Are my car speedometer readings accurate, what should I do if they are not?

Speedometer Error
Sometimes, our speedometer gives us inaccurate readings, to some degree; we can treat them as wrong readings. We are pulled over by a policeman who tells us we have exceeded the speed limit while the speedometer displays one within. This speedometer error is due mainly to variations in tire diameter, tire wear, tire temperature, pressure and vehicle load. Vehicle manufacturers usually calibrate speedometers to read high by an amount equal to the average error. This is done to ensure that their speedometers never indicate a speed lower than the actual speed of the vehicle, in an attempt to ensure they are not liable for drivers violating speed limits.
Whether your speedometer is a mechanical or an electronic one, it will need to be calibrated to compensate for any change in tire size or gear ratio. The mechanical, cable-driven speedometer can be corrected by replacing the gear or gears that are located on the output shaft of the transmission or with an externally mounted ratio adapter.
Mechanical Speedometer
For over a century the mechanical (eddy current) speedometer has been used and is still in widespread use. A rotating flexible cable usually driven by gearing linked to the output of the vehicle’s transmission inputs its signal. It was the only type commonly used until the 1980s and the appearance of electronic speedometers.
When the vehicle is in motion, a speedometer gear assembly at the rear of the transmission will turn a speedometer cable that then turns the speedometer mechanism itself. A small permanent magnet driven by the speedometer cable interacts with a small aluminum cup attached to the shaft of the pointer on the analogue speedometer. As the magnet rotates near the cup, the changing magnetic field produces “eddy” currents in the cup, which then produces another magnetic field. The effect is that the magnet exerts a torque on the cup, “dragging” it, and thus the speedometer pointer, in the direction of its rotation with no mechanical connection between them.
The torque on the cup increases with the speed of rotation of the magnet. Resistance to movement is created by the spring trying to hold the pointer at zero. At a given speed the pointer will remain motionless and pointing to the appropriate number on the speedometer’s dial. This is fixed at a preset number of revolutions per mile (usually 1,000 rpm per mile). You will need to determine what is the correct number for your application. At this point the corrections need to be made by allowing for the variations in the vehicle from stock.
Calibrating a Mechanical Cable-driven Speedometer
One of these methods can be used to determine whether your speedometer is accurate or needs to be recalibrated.
Write down the transmission type, number of teeth on the speedometer drive gear in the rear of the transmission, number of teeth on the speedometer driven gear, the rear end ratio and the diameter of the rear tire. Call or take this information to the speedometer specialist and they can use the information to build the correct ratio adapter for your vehicle.
The drive gear (at the top of the photo) for the speedometer is mounted onto the transmission output shaft within the tailhousing. White gear at the left is the driven gear on the governor. It is not part of the speedometer drive-system.
Speedometer driven gear is held in place in the tailhousing of the transmission by this O-ringed insert. Large gear is the driven gear (38 tooth) for my GM Turbo 400 transmission. Smaller gears in the foreground are for a Ford C-6. They are from the left: Blue-20, Blk-22, Red-21, Grn-22, Wht-19 and oil-discolored Wht-19 teeth replacement gears for changing the ratio. I used the Brn-18 tooth gear (not shown) in the Ford van. A number of gear sets are probably available for your application.
Speedometer drive is held into the transmission opening by a bolt and clip. Normally the cable is attached to this drive.
When a change in the combination of drive and driven gears in the rear of the transmission will not achieve the desired output speed or are more work than you want to go through to make the change then a ratio adapter will work as an alternative. It accepts input from the transmission and through a gear set outputs the desired cable revolutions.
View from this angle shows the cable-drive side of the ratio adapter that connects at the transmission and the cable connection point at the left.
Electronic Speedometers
Many modern speedometers are electronic, in designs derived from earlier eddy-current models. They consist of a rotation sensor mounted in the transmission that delivers a series of electronic pulses whose frequency corresponds to the (average) rotational speed of the driveshaft, and therefore the vehicle’s speed, assuming the wheels have full traction. The sensor is typically a set of one or more magnets mounted on the output shaft, or a toothed metal disk positioned between a magnet and a magnetic field sensor. As the part turns, the magnets or teeth pass beneath the sensor, each time producing a pulse in the sensor as they affect the strength of the magnetic field it is measuring. In more recent designs, some manufacturers rely on pulses coming from the ABS wheel sensors.
An ECU (mini computer) converts the pulses to a speed and displays this speed on an electronically controlled, analog-style needle or a digital display. Pulse information is also used to increment the odometer in place of it being turned directly by the speedometer cable.
Electronic speedometers are simpler to calibrate because it usually just involves following the calibration instructions for the given make and model of unit by manipulating the speedometers ECU (mini computer) via the adjustment controls.
Calibrating the Electronic Speedometer
Calibration of the VDO speedometer with LCD Display is a relatively simple procedure, and can be accomplished in any of three ways:
1—Automatic calibration when driving on a road with the exact distance of one-mile clearly defined; or on a dynamometer…
2—By the input of the known pulse-per-mile (kilometer) for the vehicle and sensor being used with the speedometer…
3—Using a reference point for adjustment or fine-tuning.
You gain access to the calibration functions by pressing the button on the front of the speedometer and holding it in while you turn on the ignition. As you continue to hold in the button, the display will change… scrolling through the three calibration methods and stopping on each one for about two seconds.
Calibration modes as displayed on the speedometer’s LCD
The display lists the auto-calibrate mode as “ AutOCL”; the pulse-per-mile mode as “PuLSE”; and the reference/fine-tune mode as “AdJuST”. When you see the method you wish to use, let go of the button and that function will be enabled.
1—Auto-calibration is the simplest of the three methods. However, the auto-calibration function can be used successfully only on: 1—A road with the distance of one mile actually designated by markers or 2—On a dynamometer.
After selecting the proper mode upon starting the vehicle you continue holding the button in for about three-seconds. The readout will then display “bUttOn”. When you are ready to begin your calibration run, press the button again. The display will now begin flashing the word “StArt”.
Now drive the reference distance between the markers for one mile. When you have gone exactly one mile, press the button again. If the electronic impulse rate detected by the speedometer’s microprocessor is within the calibration limits of 500 to 399,999, the rate will be shown on the LCD display. The calibration process is complete. If the electronic impulse rate is not within range, the following message: “F O.O” will be displayed and you will have to repeat the calibration process until it is in range. Most will use this method.
2—Manual Calibration with a known value. If you know the exact calibration value for the vehicle and type of sensor you are using (pulse-per-mile or kilometer-per-mile), you may use that value to manually calibrate the speedometer.
The instructions included with the speedometer give a detailed description of the procedure but the basic process involves selecting the right mode, holding the button until “PuLSE” is displayed then entering the correct value by changing the default code “50000” to the value for your specific application.
3—Manual Calibration (Fine Tuning) You can fine-tune the calibration of the speedometer’s analog display (the pointer showing miles-per-hour or kilometers-per-hour) by using speed test equipment and the “AdJuSt” function on the LCD readout. The included instructions give details about this procedure but the bottom line for this procedure is that to manually calibrate the pointer on the analog display using this method the vehicle has to be on the dynamometer at a specified speed. This method will deliver the most precise readings but most of us will not have access to this equipment and the variance in accuracy from the other methods will not be significantly different enough to warrant the cost of doing this additional step.
The VDO electronic speedometer that I am using in my roadster receives its signal from this hall-effect device via the three-wire cable that plugs into it. Plug on the speedometer end completes the connection.
I was unable to attach the hall-effect device directly to the transmission due to clearance issues between the crossmember and the device. I bought this one-to-one ratio 90° adapter to provide the needed clearance.
The ratio adapter in this case was only needed to change direction for clearance, however one with the correct ratio can be used to compensate for an out of calibration speedometer.
So you need to evaluate the particular details that apply to your vehicle and decide what course you need to take to bring its speedometer into calibration. Most speedometer errors come from a mismatch in the tire height, the rear-end gearing, or the drive-and-driven speedo gears combination in the transmission.
The vehicle’s tires can almost be thought of as a third type of gearing. For example a vehicle equipped with 295/35-18 tires, which have a circumference of 82.1 inches, would travel 82.1 inches for every complete revolution of the wheel. If the vehicle had larger tires, it would travel farther with each revolution of the wheel, which would be like a higher gear. If the vehicle had smaller tires, it would be like a lower gear.
With the gear ratios of the transmission and differential, and the size of the tires, it becomes possible to calculate the speed of the car for a particular gear at a particular engine RPM.
Therefore, it is possible to determine the distance the car will travel for one revolution of the engine by dividing the circumference of the tire by the combined gear ratio of the transmission and differential. Which method you choose will be dictated by your specific vehicle configuration. A good shop that specializes in speedometer and instrument repair is your best source of information in your quest to solve the calibration problem.
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