Five Reasons Why Your Speedometer Isn’t Working

Driving with a broken speedometer makes it difficult to abide by the legal speed limit. More than hefty fines, you can seriously put lives in danger by continuing to drive your vehicle without getting this gauge repaired.

To avoid both scenarios, it’s important to know the possible reasons why your speedometer may fail. In this article, we’ll go through possible causes, effects of driving with one, and other related queries.

Reasons Why Your Speedometer’s Not Working

The speedometer is a part of your car connected to other components via wires. So it’s usually mechanical and wiring issues that cause it to malfunction. Here are some possible reasons why your car’s speedometer isn’t working:

Faulty Vehicle Speed Sensors

The speedometer is connected to the vehicle speed sensors (VSS), so any VSS issues may lead to trouble. Usually, there is only one vehicle speed sensor and it’s mounted on the transmission’s final drive, but some platforms will use the wheel speed sensors that provide input to the ABS unit, which will network the vehicle speed to the speedometer via the instrument cluster computer. On these platforms, if the ABS light is on and the speedo doesn’t work, chase the ABS code first. Also, if the sensor input is dead, the automatic transmission may not shift out of low gear.

Internal damage to the sensor windings or to the tone wheel that spins next to the sensor can be the cause of a malfunctioning VSS. A way to tell if your speedometer is affected by a faulty VSS is if it has erratic or no readings at all.

Bad Wiring

Bad or faulty wiring can cause speedometer issues. General wear and tear is often the reason for bad wiring. However, there are cases when it short circuits due to moisture or a wet environment.

One way of telling if the problem lies within the wiring is if your speedometer reads zero even when your vehicle’s moving.

Failing Instrument Cluster

The instrument cluster on the dashboard contains all the gauges, including the speedometer. A failing instrument cluster is more common on mid-2000s vintage Chevy pickups, which tend to lose certain gauge functions due to bad stepper motors on the circuit board.

If the odometer is reading correctly but the speedometer reading is inaccurate or inoperative, it’ll be a problem with the instrument cluster rather than the speed signal.

– Richard McCuistian, ASE Certified Master Automobile Technician

If the odometer is reading correctly but the speedometer reading is inaccurate or inoperative, it’ll be a problem with the instrument cluster rather than the speed signal. Also, if the speed control still works but the speedometer doesn’t, that indicates an issue with the speedometer or cluster as well.

Usually, when one of the gauges is damaged, the entire instrument is replaced since the other ones might also show inaccurate readings.

Malfunctioning ECU

The electronic control unit (ECU) is a computer that receives input from the sensors and switches, which provides signals to the vehicle’s system to perform certain actions. A malfunctioning ECU will be unable to interpret the signals from the speed sensors and relay it to the speedometer.

Upgrading to Bigger Tires

The speedometer is calibrated according to the overall diameter of your vehicle’s tires, so replacing them with bigger tires may throw the calculation off. An increase in the tire’s circumference means a single rotation will cover more distance, resulting in slower speed readings.

If you’re replacing your vehicle’s wheels, you should always have your speedometer recalibrated to match the new tires’ diameter. On cable driven speedometers, you’d change the size of the driven gear. On newer vehicles, a professional with a high-end scan tool can usually recalibrate the speedometer based on final drive ratio and tire size.

Pro Tips are nuggets of information direct from ASE-certified automobile technicians working with CarParts.com, which may include unique, personal insights based on their years of experience working in the automotive industry. These can help you make more informed decisions about your car.

Pro Tip: You can use your GPS unit’s vehicle speed function and compare it to your speedometer reading to see how much of a difference the bigger or smaller tires have made in the vehicle speed readings. You can also hold a very even 60 mph between two mile markers and you should see exactly one minute go by if you hold your speed evenly at 60 mph during that mile.

Effects of a Broken Speedometer

A faulty speedometer will naturally affect how you drive, especially at higher speeds, since you have to be wary of your surroundings. However, more than the dangers of driving with one, there are certain issues you may encounter with a broken speedometer.

Potential Legal Consequences

Most roads and highways have speed limits If you get caught disobeying them, it’ll result in tickets and potential legal consequences. If your speedometer’s not working, you won’t be able to gauge your speed, increasing your chances of violating speed limits. You can temporarily use GPS or your smartphone if your speedometer is dead to know your speed.

Road Hazard

Not knowing your vehicle’s speed is dangerous to your passengers, pedestrians, and cars around you. Going over the speed limit increases the stopping distance needed for you to avoid crashing into other vehicles. Your risk of severe injury and vehicle damage also increases if you drive above the legal speed limit.

How a Speedometer Works

Older speedometers are driven by a cable connected to a plastic gear driven by another gear on the transmission’s output shaft. That cable spins in a flexible sheath that travels through the engine bulkhead to connect to the back of the instrument cluster. The cable has a square end that fits in a square opening in a shaft that drives a small bar magnet inside a small ferrous metal drum with a weak but calibrated spiral spring connected to the speedometer needle.

As the cable spins the magnet faster, the magnetic force on the drum will cause it to rotate a bit, always acting against the spiral spring, driving the needle away from its resting post and farther up the scale to indicate vehicle speed.

Can You Drive With a Broken Speedometer?

Driving with a broken speedometer is illegal in some states, but the rules vary for each place. You’ll get tickets for equipment violations and speeding if you’re caught driving with one. In Illinois, they also have a law in place that prohibits drivers from tampering with their speedometer.

Can You Use Your Phone as a Speedometer?

There are apps you can download on your phone that allow you to use it as a makeshift speedometer. However, it’s generally safer to have the actual instrument repaired.

There are also speedometers you can connect to your DLC port (where the scan tool plugs in) that will provide you with a digital readout. Of course, if you have issues with the speed sensors, that won’t work either. But again, if your PCM doesn’t know how fast the vehicle is going because of a sensor problem, the transmission won’t shift out of low gear.

How Much Does It Cost to Fix a Speedometer?

Fixing a speedometer costs somewhere between $150 and $250. The final bill usually depends on the parts you’ll replace, the model and year of your car, and the shop’s repair fee.

Can You Repair a Speedometer Yourself?

If you have the necessary skills and knowledge, you can troubleshoot the issue yourself. If not, then it’s recommended to have a mechanic look at your speedometer or the speed sensors instead. These components function due to their wiring systems, so you’ll need to tinker with them to have them work.

To summarize, a speedometer is an important tool that helps ensure your safety on the road. It also allows you to comply with the speed limits, helping you avoid injury and hefty ticket fines.

Where to Get New Electronic Parts for Your Vehicle

Failing electronic components due to bad wiring and defective sensors can definitely take the fun out of driving your car, truck, or SUV. If left unaddressed, these parts can even lead to more problems in the future or even cause accidents.

The good news is that here at CarParts.com, you can get all the parts you need with only a few clicks. We take the guesswork out of finding compatible products for your ride by offering direct-fit replacement parts.

Simply enter your vehicle’s year, make, and model into our vehicle selector to start browsing our catalog. You can also use our search filters to find a product according to your preferred brand, price range, quantity, and more. Check out securely in a matter of minutes, and get your order delivered straight to your doorstep in as fast as two business days.

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