Have you ever noticed a discrepancy between your vehicle's speedometer and the speed stated by your GPS system? If so, you're in good company.
But which is correct? Are car speedometers accurate? Or is your GPS displaying the correct speed?
In nearly all such cases, the car speedometer accuracy is incorrect - reporting a higher speed than the actual speed that your car is travelling at.
You might imagine that manufacturers ensure their speedometers are as accurate as possible. So why the discrepancy?
How does a car speedometer work?
A car's speedometer works by measuring the rotations of the driveshaft, axle or wheel - rather than the speed you actually travel from one point to another. The car takes the recorded rotations, applies a healthy dose of mathematics and spits out a speed.
But the accuracy of a speedometer depends on the wheels remaining the same diameter as they were when they rolled off the production line.
How do my tyres impact the accuracy of my car speedometer?
If the tyres or wheels are changed, this will impact the accuracy of the speedometer reading.
A larger diameter tyre will result in a faster recorded speed. Fitting smaller tyres or having under inflated tyres will result in a slower recorded speed.
Even a tiny change in diameter - of say, a few millimetres - will result in an incorrect speed being recorded.
With this in mind, if you’ve changed your tyres on your car recently, you may be wondering if you need to do anything to ensure the car speedometer’s accuracy is not affected.
Speedometer calibration: Does my speedometer need calibrating?
The likely error margin of the speedometer’s accuracy is factored in to how car makers calibrate their speedometers. Car manufacturers will normally deliberately calibrate their speedos to read ‘high’ by a certain amount.
How do I know if my speedometer is accurate?
To get an idea as to how accurate your speedometer reading is in comparison to the actual speed you are travelling, you can measure your speed using a sat nav, or an app such as Waze, which uses the same GPS technology as a sat nav.
How to Check Speedometer Accuracy
If you suspect your speedometer isn’t giving an accurate reading, there are a few simple ways to check it.
Follow these steps:
- Find a clear, straight stretch of road where it’s safe to maintain a constant speed.
- Set your cruise control (if fitted) to a steady speed according to your speedometer.
- Use a GPS device or smartphone navigation app (like Google Maps or Waze) to display your actual speed.
- Compare the GPS reading to your speedometer — note any difference.
- If you want to double-check, you can do a timed mile: use roadside distance markers, note the time it takes to cover a mile at a steady speed, then calculate your true speed (speed = distance ÷ time).
- Always ensure this test is done safely and legally — ideally with a passenger to help with timing or using a dash-mounted GPS.
So why is my GPS satnav accurate?
You may be wondering why is it that the speed on my satnav is four to five miles per hour slower than my car speedometer says.
GPS satnav systems calculate your speed by using satellites - and assuming the signal is strong, they should be accurate.
Some sat navs installed at manufacture are integrated with the car's own measurements to provide a more accurate speed reading.
Although in most instances a sat nav is likely to provide a more accurate reading of the actual speed you are travelling, you should rely on your speedometer in assisting you with sticking to the speed limit as this will never understate your speed.
UK Legal Requirements
Under UK speedometer regulations, all vehicles registered in the UK since 1977 have been required to have a speedometer capable of displaying speeds in kilometres per hour (kph) as well as miles per hour (mph).
The UK is the only country in Europe that still uses mph to define the speed limit. As such, many foreign registered vehicles have speedometers that only show kph and not both mph and kph.
When importing a car to the UK from another country, it is often necessary to change the speedometer from kilometres per hour (kph) to miles per hour (mph) due to the speed limit being set in mph.
Under UK law — which is based on a European Union standard — UK speedometer regulations also outline how accurate speedometers must be.
Key UK speedometer requirements include:
- The speedometer must never underreport a vehicle’s speed.
- The speedometer must not overreport by more than 110% of true speed plus 6.25mph.
- Speeds must be displayed in both mph and kph for vehicles registered in the UK since 1977.
- Imported vehicles must have speedometers converted to mph if they originally display only kph.
So if your true speed is 40mph, your speedometer may legally read up to 50.25mph — but it can never read less than 40mph.
Car manufacturers speedometer calibration UK
In order to stay within the law, and make sure that their speedometers are never showing less than true speed under any foreseeable circumstances, carmakers calibrate their speedometers to slightly overreport their vehicles' speeds.
So, in answer to are speedometers accurate, it is clear that the calibration of speedometers by car manufacturers of course means many motorists are travelling slower than they think - which is arguably good news for avoiding accidental speeding tickets and for making our roads safer.
You may well find your sat nav’s GPS is a more accurate determinant of your speed - although you should stick to your car’s speedometer reading to be on the safe side to ensure you stay within the speed limit.
Technological developments also look set to change the way we drive, and the speeds we can travel.
Mandatory speed limiters
After several years in the pipeline, Intelligent Speed Assist (ISA) technology is set to become mandatory on all new cars in Northern Ireland from July of this year.
Every new car or van sold after 7th July must have a mandatory speed limiter fitted. Unsold, new cars which have already been manufactured are also required to be retrofitted with speed limiters.
Whilst the new law applies to Northern Ireland and will not be mandated in England, Scotland or Wales, it is likely ISA technology will be included on new cars manufactured for any of the UK market.
Speed limiters detect the speed limit on any given road and automatically brake and slow down the car.
Speedometer vs. GPS: Which is More Accurate?
In almost all modern cars, the speedometer will slightly overstate your true speed. For example, if your speedometer reads 70mph, your actual speed might be closer to 65mph according to a GPS device.
This is intentional. Manufacturers calibrate speedometers to ensure they never show a speed lower than the true speed, as required by law. By contrast, GPS devices calculate speed based on your exact position over time, so they tend to be more precise — assuming a good satellite signal and no signal loss in tunnels or dense urban areas.
In short, your GPS is usually closer to your true speed, but for legal purposes you should always follow your speedometer to avoid unintentionally speeding.
FAQs
If my satnav shows I’m going slower than my speedometer, which do I believe?
This is very common — your car’s speedometer will almost always read slightly higher than your actual speed due to legal requirements to avoid under-reporting. In contrast, your satnav calculates speed using GPS, which is generally more precise.
If you want to check which is closer to the truth, compare your speedometer with a GPS reading on a straight, clear road at steady speed. While your satnav will usually be more accurate, always stick to your speedometer for legal and safety reasons, as it ensures you do not accidentally exceed the speed limit.
