Cruising down the highway with the windows rolled up and sweet tunes oozing through your premium sound system. That’s the good life. Glancing out through the window, you can see things fly by – but you can’t tell how fast.
It’s proven that humans can’t estimate speed accurately, especially if the acceleration is gradual. This is why cars need a speedometer.
Speedometers show a relatively accurate display of how fast your car is moving at a specific time.
Traditional speedometers were a mechanical clock-like dial, while modern cars have digital speedometers that mimic the classic speedo or display your speed as a number on the screen.
Before figuring out why speedometers fail, we have to look at the two main types of speedos, how they work, and their possible failure points.
How Mechanical Speedometers Work (Cable-Driven Speedo)
Mechanical speedometers date back to 1902 when an inventor, Otto Schulze, invented them.
The unit uses magnetic eddy currents to display speed on your dash. To do this, the cable speedometer unit has a drive cable around the transmission shaft. As the shaft spins, the drive cable and the mandrel it’s attached start to rotate.
The other side of the drive cable attaches to a spiral gear. The opposite side of the assembly has a magnet in a speed cup. As the helical gear spins the magnet, it creates a spinning magnetic field that in turn drags on the speedometer’s needle.
A hairspring balances out the motion ensuring that the speedometer’s needle deflection is proportional to the vehicle’s speed.
Mechanical Speedometer Failure Points
Mechanical speedos are robust and, frankly, hard to break. The only time the speedo will stop working is if there’s a direct impact on the speedometer’s mechanical parts.
A damaged cable, spoiled gears, or a damaged gauge cluster that holds the speedo needle affixed to one-stop will stop your speedometer from working.
Start troubleshooting by checking out your dashboard and ensure that nothing is obstructing the speedo needle.
After that, take out the speedometer and confirm that the cable is attached. After that, you can trace everything back to the driveshaft and ensure that you don’t see any visible damage.
ProTip: Get a professional with experience as they can identify failures faster than you can
If you or the mechanic can’t find a problem, consider replacing the entire system with a complete system from a similar vehicle.
Luckily most modern cars have digital speedos. This means you can get replacement pieces from wrecked or junked vehicles.
How an Electronic Speedometer Works
If your car has an electronic speedometer, it gets information from a dedicated speed sensor. Again, the sensor gets its data from the transmission shaft in your vehicle.
The sensor has a toothed metal disc, a magnetic coil, and a stationary detector. As the driveshaft spins, the toothed metal disc momentarily blocks the magnetic field from getting to the sensor.
These interruptions are detected as pulses transmitted to the vehicle’s computer, which in turn calculates how fast the car should be moving for the given RPM.
The information is then used to move a traditional analog dial to the correct position or display the information on a digital display.
Electronic Speedometer Failure Points
Electronic speedometers are still reliable but have a couple more failure points than a cable speedo. Some of the most common failures include:
A Damaged or Faulty Speed Sensor
Since the speed sensor is the primary data source, a problem with the sensor will throw your entire system out of sync. Luckily, most cars with digital speedometers use information from the sensor to control other vehicle functions, for instance, cruise control.
If this is the case, chances are your car’s ECU throws an error code that lights up your check engine light.
Nonetheless, the ECU could choose to disregard information from the speed sensor. It could also be getting the reading it would expect where the car isn’t moving.
Compromised Instrument Cluster
Everything else could be working fine, but your speedo gauge or display has an issue. This is common in vehicles that use a mechanical speedometer display.
A burned-out motor, a stuck needle, or dislodged terminals could stop it from reacting to the information coming from the car’s computer.
Since mechanical displays only show speed, they’re more prone to such failure. Your odometer will still work. Only the speedometer fails.
Instrument cluster failure in cars with digital speedometers will be more noticeable. It won’t display anything and will remain dark when you switch on the vehicle.
Some Fault in the System’s Wiring
Since all the communication between the speed sensor, the ECU, and the gauge on the cluster is all electronic, any fault in the wiring system could throw the system off.
Troubleshooting this could be a bit hard for a hobbyist, especially if it’s on a car you daily drive. A mechanic or someone with experience will identify the problem faster and fix it.
However, you can track down the wiring and identify any broken wires, worn-out insulation, or loose terminals if you have time.
A Blown Fuse
If you are lucky, the wiring problem will cause a short circuit blowing a fuse. Replacing your car’s speedometer circuit fuse might fix the problem for a while – especially if the short circuit was momentary.
If you keep blowing fuses, have the entire system troubleshot.
At other times, the wiring could be cut. This won’t blow any fuses; hence you will have to troubleshoot the entire system manually.
A Failed ECU
Since the ECU is in charge of interpreting the speed data and telling the gauge cluster what to display, a problem in the ECU could lead to a failed or wrongful reading.
A failed ECU will cause additional problems.
Note that some cars have multiple dedicated computers. Chances are, the module controlling your car’s speed reading doesn’t handle other car functions that could lead to immediate and visible failures.
What to Do if the Speedometer Stops Working
Have your speedometer system inspected and fixed as soon as you realize it’s not working. This is crucial, especially if your car has cruise control.
Additionally, a speedometer provides crucial driving feedback that helps drivers tune their reflexes and mind accordingly. You will subconsciously be more alert and even slow down when cornering if you can tell how fast your car is moving in advance.
Can You Still Drive Your Car With a Failed Speedometer?
While you can technically still drive a car with a failed speedometer, the experience isn’t safe as you can’t tell how fast you’re going.
You might make wrong calculations and cause accidents.
Moreover, if you are pulled over for overspeeding, you can’t use your failed speedometer as an excuse to get off the ticket.
Using Your Car’s GPS to Tell Your Speed
If you use Google Maps, Waze, or other driving apps on your smartphone, you’ll have noticed they use GPS and the phone’s accelerometer to deduce how fast you’re moving.
Sometimes, the estimate is pretty close to what your car is doing. While this can be a makeshift solution, don’t over-rely on it. There is no guarantee that the readings you get off your smart device are as accurate as they ought to be.
Moreover, your car can’t tap into your smartphone, and if it can’t tell how fast it’s going, features that rely on speed data like cruise control, traction control, and even fuel injection might not work well (or even work at all)
How to Fix a Misreading Speedometer
While your speedometer gets its speed reading from figuring out how your transmission shaft is moving and using other constants to come up with a final figure, the reading could be wrong if one of the constants becomes variables.
Common assumed constants when calculating speed include:
Your Car’s Tire Size
Car tires are the most popular culprits in throwing your speedometer off the mark. A change in your tire’s outside diameter affects how much distance they cover per revolution.
This will, in turn, affect how fast your car moves per fixed set of revolutions as read on the transmission shaft.
Installing bigger than recommended wheels will make your car move faster than indicated on the speedometer.
Smaller tires will make the car move slower than the speedo says.
The easiest way to avoid this is by sticking to recommended tire size. If you must upgrade, for instance, after lifting your offroading rig, remember to tune your speedometer to compensate for the bigger tires.
Your Final Drive Ratio
The final drive ratio determines how many revolutions get from a specific transmission shaft RPM. It affects torque and a car’s top speed.
Changes in the final drive ratio are common in offroading enthusiasts and trucks with a solid rear axle differential.
If you change your final drive ratio, you will have to adjust your speed calculations to accommodate the variation.