GPS vs Car Speedometer: Why Your Dash Reads Higher

Your car’s speedometer reads higher than a GPS speedometer because it’s built to. Vehicle rules in Europe, the UK and many other countries say a speedometer may never show less than your true speed, so carmakers aim a few percent high, and worn tires push the reading up a little more. GPS measures how fast you’re moving over the ground, so on an open road at a steady speed it’s usually the closer number. A dash showing 70 mph while the phone says 66 or 67 is normal.

The gap only becomes a problem when it runs the other way. If your dash reads lower than GPS, something has changed, usually the tire size, and you may be speeding without knowing it. Here’s where each number comes from, how far apart they’re allowed to be, and when GPS is the one that’s wrong.

How much can a speedometer legally read high? #

Most carmakers design to UN Regulation No. 39, the speedometer rule used for vehicle approval in the EU, the UK and many other countries. Its tolerance only runs one way:

0 ≤ indicated speed − true speed ≤ 10% of true speed + 4 km/h

The left half means the speedometer may never read low. The right half caps how far it may read high.

Those are ceilings. Most cars read well inside them, typically 2 to 5% high at highway speed. The US isn’t bound by the UN rule, and the federal accuracy rule on the books, 49 CFR 393.82, covers trucks and buses (within 5 mph at 50 mph). Carmakers build to global designs, though, so US-market cars tend to read a little high as well.

Why does a car speedometer read high? #

A car never measures its speed over the road. It counts how fast the wheels or the gearbox output turn and multiplies by the distance one tire revolution is supposed to cover. Anything that changes that distance changes the reading.

The built-in margin #

No speedometer can be exact for every tire, load and pressure a car will run on. Reading low is illegal and reading high isn’t, so manufacturers aim the whole spread of likely errors above true speed.

Tire wear #

A worn tire is smaller, so it turns more times per mile and the speedometer reads higher. A common 205/55R16 tire is about 632 mm across when new. Wearing the tread from about 8 mm down to the 1.6 mm legal minimum in the UK and EU takes roughly 13 mm off the diameter, around 2%. That’s about 2 km/h of extra error at 100 km/h by the time the tires are due for replacement.

Tire size changes #

Smaller tires than stock push the reading higher still. Taller tires, common on lifted trucks and with some wheel upgrades, make the speedometer read low. That’s the dangerous direction: a tire 3 to 4% taller than stock can cancel the factory margin, and anything bigger leaves you going faster than the dash says. The odometer is thrown off by the same percentage. Our guide to calculating speedometer error after new tires works it out from the sidewall size.

Pressure, load and worn parts #

Under-inflated tires and heavy loads squash the tire slightly and nudge the reading up, usually by under 1%. Cable-driven speedometers in older cars and bikes add mechanical slop: a worn cable or a tired spring makes the needle bounce and lag. A failing wheel-speed sensor gives jumpy or wildly wrong readings rather than a steady offset.

Is a digital speedometer more accurate than an analog one? #

No. In a modern car both displays are fed by the same speed signal, which already contains the tire assumption and the factory margin. An analog needle is usually moved by a small electric motor rather than a cable, and you read it against marks every 5 or 10 mph, so it’s easy to misread by a mile or two per hour. A digital readout shows a crisp whole number that looks precise, but it’s the same signal, sometimes smoothed or rounded.

Digital is easier to read at a glance. Neither is closer to your true speed.

How does a GPS speedometer measure speed? #

Your phone’s GPS chip measures the Doppler shift in the satellite signals, the tiny change in frequency caused by your movement, and turns it into a velocity. That’s why GPS speed can be accurate even when your position is a few metres off. Tires, gearing and wheel slip don’t enter into it. When a phone doesn’t report a Doppler speed, an app has to divide the distance between two fixes by the time between them, which is noisier.

Speedometer GPS uses the phone’s own speed whenever the phone reports one, takes a fresh fix every second and shows the average of the last five readings, so the number reads steady instead of twitching. At a steady speed with a good fix, that’s typically within a couple of km/h of true ground speed.

When is the GPS speed wrong? #

GPS is only as good as its view of the sky. Expect trouble when:

• You lose the sky. Tunnels, parking garages and covered bridges block the signal completely.
• Signals bounce. Tall buildings reflect satellite signals, which shifts your apparent position and makes speed jump.
• You accelerate or brake hard. Smoothing makes the display trail your real speed by a second or two. Our GPS speed lag guide explains how big that gets.
• The fix is weak. Right after you open the app, or with the phone in a cup holder or glovebox, the receiver may be working with too few satellites.

Speed itself isn’t on that list. GPS doesn’t get less accurate as you drive faster, which surprises people who assume the gap at 70 mph must be a GPS problem; GPS speed accuracy at high speed covers what’s really going on at highway pace.

Speedometer GPS shows a colour-coded pill under the readout with the current accuracy radius: EXCELLENT within 5 m, GOOD within 10 m, FAIR within 20 m and POOR beyond that. Fixes worse than 20 m are shown but not used, and if no valid fix arrives for five seconds the readout drops to 0 and says SEARCHING rather than freezing on an old number. Only compare against your dashboard when the pill says GOOD or EXCELLENT.

Which one should you trust? #

For staying legal day to day, drive by the dashboard. Its high bias works in your favour: keep the needle at the limit and you’re almost certainly at or a little below it. GPS is the tool for finding out how big that margin is.

How do you check your own car? #

The quick version takes one drive. Mount the phone and open the app before you leave, then on a flat, straight road hold a steady speed with cruise control and have a passenger read both numbers after 15 seconds or so. The error is (dash − GPS) ÷ GPS × 100, so a dash of 70 against a GPS 67 is 4.5% high. Repeat at two or three speeds.

Driving alone, don’t read the phone. Set the app’s speed alarm to a round number before you go, such as 60 mph, cruise a little below it by the dash, and raise the cruise speed one step at a time. The dash reading when the phone buzzes tells you your margin. The full method, including a stopwatch-and-mile-marker test and what to do with the result, is in how to check and calibrate a speedometer with GPS.

A dash that reads 1 to 5 mph over GPS at highway speed is normal. One that reads under GPS needs fixing, and the tire size is the first thing to compare against the placard in the driver’s door jamb. Until it’s corrected, set the speed alarm at the posted limit. GPS doesn’t care about tire size, so the phone buzzes at your true limit whatever the dash shows.

Does the gap affect your odometer and fuel economy? #

Only partly. The current version of the same UN regulation requires the odometer to stay within ±4% of true distance, with no deliberate upward margin, so trip distance often matches GPS more closely than speed does. Tire wear and size changes move the odometer by the same percentage as the speedometer, though. If yours over-counts by 3%, a trip computer showing 30 mpg over 300 miles is really about 29 mpg over 291 miles, and lease mileage and service intervals drift the same way. We compare the two distances in why GPS trip distance doesn’t match your odometer.

Frequently asked questions #

Is GPS more accurate than a car speedometer? #

At a steady speed on an open road with a good satellite fix, yes. GPS measures ground speed directly, while a car speedometer is set to read high and depends on the tire size it was programmed for. GPS is worse in tunnels, between tall buildings and during hard acceleration.

Is it normal for my speedometer to read 5 km/h higher than GPS? #

Yes. At highway speed a gap of 3 to 5 km/h (2 to 3 mph) is common and well inside UN Regulation 39, which allows up to 10% plus 4 km/h. If the gap is much bigger, or the dash reads lower than GPS, check tire size and pressure.

Do bigger tires make my speedometer read faster or slower? #

Slower. A taller tire covers more ground per turn, but the car still counts each turn as the original distance. Your speedometer and odometer both read low, which can put you over the limit while the dash says you’re at it.

Why does my GPS speedometer drop to zero in a tunnel? #

GPS needs a view of the satellites, and a tunnel blocks it. A well-behaved app shows zero or a searching message rather than repeating your last speed. Your car’s speedometer keeps working because it reads the wheels.

Can a GPS app prove I wasn’t speeding? #

Don’t count on it. Enforcement uses calibrated radar, lidar or camera systems, and a phone app isn’t a certified instrument. Use GPS to learn your car’s margin and drive accordingly, and if you’re contesting a ticket, talk to a local traffic lawyer.