Why doesn't the speed shown on my car's speedometer and sat-nav always match? (2024)

FAQs

Why doesn't the speed shown on my car's speedometer and sat-nav always match? ›

“It is the change in elevation, relative to the GPS satellites circling above, that results in the error. Horizontal bends do not affect it. “In theory, a clever GPS device could account for the road steepness and adjust the displayed speed so it is more accurate.

Speedometers are allowed to overstate speed by up to 10 per cent, but never to understate it. Speedos therefore often indicate a higher speed than GPS. At a steady speed on a straight road, sat-nav is likely to be more accurate than the speedo.

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.

Start by inspecting the fuses related to your instrument cluster and speedometer. A blown fuse can often be the culprit. Inspect wiring and connectors. Examine the wiring and connectors associated with the speed sensor, speedometer cable, or VSS for any visible damage.

To comply with international norms, speedometers cannot show a speed lower than the car's actual speed. They tend, in fact, to display a higher number. That's the main reason why there may be a difference between the speed indicated by the car and the one that appears on navigation apps.

“When the manufacturer carries out speed calibration tests, they're based on brand new tyres. “But over time, as the tyres experience normal wear and tear, they get smaller in circumference. This changes the accuracy of the reading of the speedometer – again showing a higher speed than the actual speed.

A vehicle's speed can be made by measuring the time it takes to travel one mile. The best way to conduct the test is to set cruise at one speed and measure the time it takes to travel between two green mile markers on interstate or other US/state highways.

Tire size and speedometer accuracy are directly linked to each other. Up-sizing, or installing a taller tire, will lead to a speedometer reading that is slower than your actual speed.

Before turning the vehicle on, key off, engine off, press and hold the trip reset button on the dial, start the vehicle, and continue to hold the button. The needle will jump to half-scale, then to its maximum reading. Once you're at full scale, release the button, and we've staged the speedometer for calibration.

The erratic or inaccurate speedometer reading is one of the common signs of a bad speed sensor. If the reading of your speedometer is constantly going up and down, then there is a chance of a failed vehicle speed sensor. Your vehicle will continue to run without a working speedometer.

How do I reset my speed sensor? ›

Assuming the speedometer itself isn't broken, it can read inaccurately if the tires are a different size than it was calibrated for. Larger tires will cause the speedometer to give a lower reading, and smaller tires will cause a higher reading.

The thing is, switching out the battery or even jumpstarting a vehicle can cause an electrical spike inside of the vehicle, which basically can cause a speedometer to go berserk.

Weather and general environment. Any obstacles in between your phone and the satellites — be it clouds, trees, buildings, or tunnels — blocking or reflecting the signal, will create positioning errors and delays. This is why you often see degraded GPS accuracy in downtowns.

The speed sensor is less prone to error (variability) which means speed and distance will be more accurate.

The addition of a speedometer in your navigation shows you how fast you're driving on the road. Important: Speedometers shown in the Google Maps app are for informational use only. Make sure to use your vehicles's speedometer to confirm your actual driving speed.

In short, when comparing GPS vs. odometer accuracy, GPS devices win by a landslide since the tire condition and similar factors do not determine whether the captured mileage will be accurate.

GPS positions are accurate 95% of the time to +/- 13 metres and updates can be received every second.

Your car's speedometer rarely tells the truth. If you've ever used a sat-nav app on your phone you may have noticed that the speed it shows you're travelling at is slightly higher than that shown on your car's speedometer – and the faster you go the more inaccurate it tends to be.