To check your speedometer, hold a steady speed on a straight, open road, compare the dashboard with a GPS speed reading, and repeat at two or three speeds. The error is (dash − GPS) ÷ GPS × 100. A dash that reads 2 to 5% high is normal and legal almost everywhere. A dash that reads low is the one to fix, and the usual reason is a tire change: taller tires make the speedometer under-read and the odometer under-count by the percentage the diameter grew.
Correcting it depends on the car. Factory speedometers are fixed by returning to the stock tire size, reprogramming the tire size in a control module, or fitting a correction module. Older cable-driven gauges need a different drive gear, and many aftermarket gauges have a drive-calibration mode where you drive a measured distance, often two miles, and press a button.
None of this needs you to look at your phone while you drive. Every method below uses a passenger, an alarm you can hear, or a result you read after you’ve parked.
Why is GPS a fair reference? #
Your speedometer infers speed from wheel or gearbox rotation and a programmed tire size, and it’s designed to read a little high so it never shows less than true speed. GPS measures your movement over the ground from the Doppler shift of satellite signals, so tire size, wear and gearing don’t affect it. At a steady speed with a good fix, phone GPS is typically within a couple of km/h of true speed. Why the two disagree, and how far apart they’re allowed to be, is covered in GPS vs car speedometer.
GPS has weak spots of its own. It trails by a second or two when your speed changes, drops out in tunnels and wanders between tall buildings. That’s why every test below uses a steady speed on an open road.
Before you start #
- Pick the road. A straight, fairly flat highway with open sky, light traffic and no tunnels or long overpasses, where holding a steady speed is legal and safe.
- Check the tires. Set the pressures to the placard in the driver’s door jamb, with the tires cold. Compare the size on the sidewall with the placard size while you’re there.
- Set the phone up while parked. Mount it where it can see the sky through the windshield, open Speedometer GPS and wait for the GPS pill under the readout to show GOOD (within 10 m) or EXCELLENT (within 5 m). Keep the app on screen, because it doesn’t track in the background.
- Use cruise control if you have it. It holds a steadier speed than your foot.
How to test your speedometer with GPS #
Pick whichever method suits who’s in the car.
Method 1: a passenger reads both numbers #
- Hold the dash at a round speed: 50, 60 and then 70 mph, or 80, 100 and 120 km/h where that’s legal.
- Give each speed at least 15 seconds. The app averages the last five one-second fixes, so it needs a few seconds to settle.
- Your passenger notes the dash and GPS readings at each step.
- Work out the error for each speed.
Method 2: alone, with a speed alarm #
- Before you leave, set the app’s speed alarm to a round number at or below the road’s limit, such as 60 mph or 100 km/h.
- On the test road, set cruise control a little under that number by the dash.
- Raise the cruise speed by 1 mph or 1 km/h at a time, holding each step for about 15 seconds.
- When the phone buzzes and beeps, note the dash speed. An alarm set to 100 km/h that goes off at an indicated 104 means your speedometer reads about 4% high.
Method 3: read the trip after you park #
- Press Start before you leave.
- On the test road, hold each dash speed for 30 to 60 seconds, and keep those steady stretches as the only time you drive at those speeds.
- Park, press Stop, then Save, and open the trip. The speed chart shows a flat line for each steady stretch, and you read the GPS value off each one. If you tested one speed and it was the fastest part of the drive, the trip’s max speed is your true speed at that dash reading.
Max speed in Speedometer GPS comes from the smoothed value, so one glitchy sample can’t fake a high number.
Method 4: mile markers and a stopwatch #
No phone needed. Hold a steady dash speed and have a passenger time the gap between highway mile markers. True speed in mph is 3,600 divided by the seconds per mile.
| Seconds per mile | True speed |
|---|---|
| 55 | 65.5 mph |
| 58 | 62.1 mph |
| 60 | 60.0 mph |
| 62 | 58.1 mph |
| 64 | 56.3 mph |
| 66 | 54.5 mph |
Markers aren’t always placed with survey precision, so time five miles rather than one. At a true 60 mph, five miles takes exactly 300 seconds.
What do your results mean? #
A typical car on part-worn original tires might look like this:
| Indicated | GPS | Error |
|---|---|---|
| 50 mph | 48.0 mph | +4.2% |
| 60 mph | 57.8 mph | +3.8% |
| 70 mph | 67.5 mph | +3.7% |
A steady percentage like that is normal. If the percentage jumps around between speeds, your speed wasn’t steady or the fix was weak, so repeat the run.
| Dash vs GPS at 60 mph | What it usually means |
|---|---|
| 1 to 4 mph higher | Normal design margin |
| About equal | Fine; new or slightly larger tires have often used up the margin |
| 5 or more mph higher | Smaller or badly worn tires, low pressure, or the wrong size programmed |
| Lower than GPS | Taller-than-stock tires, a gearing change or wrong programming. You may be speeding without knowing it |
Why do bigger tires change your speedometer? #
Your speedometer counts wheel or gearbox rotations and multiplies by the tire circumference it was programmed with. Put a bigger tire on and each turn covers more road than the car thinks, so it shows less speed than you’re doing. The odometer runs off the same signal, so it under-counts distance by the same percentage.
New tires of the same size shift the reading too, because fresh tread is taller than worn tread. A car tire worn from new down to the legal minimum loses around 13 mm of diameter, roughly 2% on a 205/55R16. That’s too small to bother correcting.
How to work out tire diameter from the sidewall #
A metric size such as 265/70R17 has three numbers you need:
- 265 is the tread width in millimetres.
- 70 is the aspect ratio: the sidewall is 70% as tall as the tire is wide, so 265 × 0.70 = 185.5 mm.
- 17 is the wheel diameter in inches, or 431.8 mm.
Overall diameter is the wheel plus two sidewalls: 431.8 + (2 × 185.5) = 802.8 mm, about 31.6 inches.
Off-road “flotation” sizes such as 35x12.50R17 give the diameter directly as the first number. Treat it as nominal, because real tires often measure a little under the number on the sidewall.
The formula for speedometer and odometer error #
- Ratio = new diameter ÷ old diameter
- True speed = speedometer reading × ratio
- True distance = odometer distance × ratio
A ratio above 1 means the speedometer reads low and the odometer under-counts. Below 1, both read high.
| Diameter increase | Dash shows 30 | Dash shows 50 | Dash shows 70 |
|---|---|---|---|
| +3% | 30.9 | 51.5 | 72.1 |
| +5% | 31.5 | 52.5 | 73.5 |
| +7% | 32.1 | 53.5 | 74.9 |
| +10% | 33.0 | 55.0 | 77.0 |
The numbers work the same in km/h. On a 28-inch tire, one extra inch of diameter is about 3.6%; on a 33-inch tire the same inch is about 3%.
Worked examples for common tire changes #
| Change | Old diameter | New diameter | Ratio | Indicated 60 mph is really |
|---|---|---|---|---|
| SUV: 225/65R17 to 245/70R17 | 28.5 in | 30.5 in | 1.069 | 64.2 mph |
| Truck: 265/70R17 to 285/75R17 | 31.6 in | 33.8 in | 1.070 | 64.2 mph |
| Truck: 265/70R17 to 315/70R17 | 31.6 in | 34.4 in | 1.087 | 65.2 mph |
| Car: 205/55R16 to 225/45R17 | 24.9 in | 25.0 in | 1.004 | 60.2 mph |
The last row is what a well-chosen “plus one” wheel upgrade looks like: the wheel grows an inch, the sidewall gets shorter, and the overall diameter barely moves. The first three are typical lift-kit and off-road upgrades, and each leaves the speedometer 4 to 5 mph low at highway speed.
The odometer follows the same ratio. On the SUV in the first row, every 100 miles the odometer logs is really about 107. Over a 12,000-mile lease year, that’s around 830 miles the car never counted.
Why measure as well as calculate? #
The calculation tells you how much the tire change moved the reading. It doesn’t know where the speedometer started. Most leave the factory reading a few percent high on purpose, so the net result can surprise you. Suppose yours read 3% high on stock tires and you fit tires 6.9% taller. The dash now reads about 3.6% low: at a true 65 mph it shows about 62.6. Neither number alone gave you that. Real tire diameters also vary a little by brand, and a loaded tire is slightly squashed. Run one of the GPS tests above and use measured ratio = GPS speed ÷ dash speed.
For the odometer, drive at least 10 miles on a highway and compare the car’s trip meter with a GPS trip or with the mile markers. Do it at highway speed: Speedometer GPS drops movements under 5 m between fixes to avoid drift, so it under-counts badly below about 11 mph (18 km/h). Our odometer accuracy guide covers the longer test.
How do you correct a speedometer? #
- Go back to the stock tire size. If the error appeared with new wheels or tires, this is the simplest fix.
- Reprogram the tire size or axle ratio. Many modern vehicles store these in a control module. Depending on the make, a dealer, an independent shop with the right scan tool, or a handheld programmer for your model can change them. Some makes accept only sizes offered from the factory, so ask before you buy the tires. If you regeared along with the bigger tires, enter the axle ratio too.
- Fit a correction module. For vehicles that can’t be reprogrammed, an inline calibrator between the speed sensor and the computer rescales the signal.
- Change the drive gear. Older cars with a cable-driven speedometer are corrected with a driven gear with a different tooth count in the transmission or transfer case.
- Motorcycles. Bikes that read speed from the gearbox output are thrown off by sprocket changes as well as tire size, and an inline speedo corrector is the usual fix.
The speed signal often feeds the transmission, ABS and stability control as well as the gauge, so have it done properly. Correcting the tire size can clear up odd shifting at the same time. Some tire shops that fit lift kits can do the programming when they fit the tires, which saves a second visit.
The 2-mile drive calibration on aftermarket gauges #
Many programmable aftermarket speedometers calibrate themselves by counting sensor pulses over a known distance. The usual pattern is to enter calibration mode (often by holding the gauge’s button while switching the ignition on), drive a precisely measured distance, commonly one or two miles depending on the brand, and press the button again at the end. Follow your gauge’s manual, since the steps vary.
That routine belongs to aftermarket gauges. On factory instrument clusters, holding the trip-reset button doesn’t recalibrate the speedometer.
The hard part is knowing exactly where the two miles end without staring at a screen:
- On an earlier drive, record the stretch in a GPS app and pick fixed landmarks at the start and at 2.00 miles. Check them afterwards, parked, on the saved trip’s route and distance.
- For the calibration run, press the gauge’s button at the first landmark and again at the second, with your eyes on the road.
- Test the result with GPS at a few speeds, as above.
Do the recording run at normal road speed. At highway speed each step between one-second fixes is 25 m or more, so the app’s 5 m drift filter doesn’t come into it.
What happens if you don’t correct it? #
You can live with a known error, but it’s easy to forget:
- Speeding without knowing. A 7% under-read means 75 mph when the dash shows 70.
- Mileage records. Service intervals come round later than the car thinks, and warranty, lease and resale mileage no longer reflect true miles.
- Fuel economy and range. The trip computer uses the same distance signal, so it under-states both.
- Gearing and driver aids. Taller tires act like taller gears, and ABS and stability control work from wheel-speed sensors that assume the programmed size.
A common rule of thumb is to stay within about 3% of the original overall diameter when choosing new tires or wheels. On all-wheel-drive vehicles, keep all four tires the same size and similarly worn.
Checking a used car on a test drive #
The same check works on a car you’re thinking of buying. If a previous owner fitted bigger wheels and tires, the speedometer under-reads and the odometer has under-counted miles ever since, and neither shows on the dash. Ride as a passenger for part of the test drive, or bring someone who can read the phone while you drive, and compare the tire size with the door placard.
A GPS check can’t catch odometer rollback. For that, rely on a vehicle history report, dated service records, and wear on the pedals, seat and steering wheel that matches the mileage shown.
After calibrating, check again #
Aim for a speedometer that matches GPS or reads slightly above it at highway speed. A mile or two per hour high is a harmless safety margin; low is not. The odometer moves with the same correction.
Until the car is fixed, a GPS speed alarm is a useful backstop. Set it in Speedometer GPS to the posted limit before you leave, and the readout turns red and the phone buzzes when your true speed passes it, then again every 10 seconds until you slow down. Our guide to speed alert apps covers what limit to pick.
Frequently asked questions #
How accurate should my speedometer be? #
Under UN Regulation No. 39, used in the EU, the UK and many other countries, a speedometer may never read below your true speed and may read up to 10% plus 4 km/h above it. Most cars land well inside that, a few percent high. In the US, the federal rule in 49 CFR 393.82 covers buses and trucks: within 5 mph at 50 mph.
Is a 5 mph difference between GPS and my speedometer normal? #
At 60 or 70 mph it’s at the high end of normal. Check tire pressure and size first, since worn or smaller tires make the speedometer read higher. If the gap stays large with the right tires at the right pressure, have a dealer check the programming.
Do bigger tires make you go faster than the speedometer says? #
Yes. Each turn of a bigger tire covers more ground, so at the same dash reading you’re moving faster, by the same percentage the diameter grew. Going from about 31.6-inch to 33.8-inch tires means about 64 mph at an indicated 60.
Can I calibrate my speedometer myself? #
You can always check it yourself with GPS. Correcting it depends on the car: aftermarket gauges usually have a drive-calibration mode, older cable-driven speedometers need a different drive gear, and modern factory speedometers are corrected by reprogramming the tire size or fitting a calibrator, which may need a scan tool or a shop.
Does the GPS app need calibrating too? #
No. GPS speed doesn’t depend on anything you could adjust, so there’s no offset to set, and Speedometer GPS doesn’t have one. What you can control is the fix: phone placement, a clear sky and waiting for a strong signal. Our guide to calibrating a GPS app covers that.