In most GPS speedometer apps, switching between mph and km/h is a single tap, and it doesn’t change accuracy at all. The phone measures your speed in metres per second and the app converts it for display, so mph, km/h and knots are the same measurement written three ways. In Speedometer GPS, tap the unit button to cycle through km/h, mph and knots; distance, altitude and your saved trips switch along with it, and the app remembers your choice.
The things that do go wrong when you change units are the speed limit alarm, comparisons that look like they don’t add up, and reading the wrong label. Here’s how to switch cleanly and sort out the mismatches.
How do I switch units in a GPS speedometer? #
In Speedometer GPS:
- Tap the unit button on the main screen. Each tap moves to the next unit: km/h, then mph, then knots.
- Or open Settings > Units and pick kilometres per hour, miles per hour or knots.
- Re-pick your speed alarm in the new unit if you use one (more on that below).
Do it while you’re parked. Here’s what each setting shows:
| Unit | Speed | Distance | Altitude |
|---|---|---|---|
| km/h | km/h | metres below 1 km, then kilometres | metres |
| mph | mph | feet below 0.1 mile, then miles | feet |
| knots | knots | metres below 0.1 nautical mile, then nautical miles | metres |
Short distances switch to feet or metres so the start of a trip reads in useful steps instead of 0.01 miles.
Does switching from mph to km/h change accuracy? #
No. GPS speed is measured in metres per second from the satellite signals, and every unit is a conversion of that number.
| Conversion | Value |
|---|---|
| 1 mph in km/h | 1.609344 (exact) |
| 1 km/h in mph | 0.621371 |
| 1 knot in km/h | 1.852 (exact) |
| 1 knot in mph | 1.150779 |
| 1 m/s in km/h | 3.6 (exact) |
Those aren’t rounded estimates. The international mile has been defined as exactly 1,609.344 metres since 1959, the figure NIST publishes, and a nautical mile is exactly 1,852 metres.
What you might notice is resolution. The readout shows one decimal place, and 0.1 km/h is a smaller step than 0.1 mph. The same GPS wobble also looks bigger in km/h: ±2 km/h is only ±1.2 mph. That’s the display, not accuracy. What actually affects accuracy is the signal: tunnels, tall buildings, tree cover and where the phone sits.
How does the app convert units? #
Speedometer GPS takes the metres per second its receiver reports, turns it into km/h, averages the last five one-second readings so the number doesn’t twitch, and converts only when it draws the screen. Everything it stores stays metric.
That design matters more than the precision of any single factor:
- Toggling km/h to mph to knots and back never changes the underlying numbers, so nothing compounds.
- Saved trips and the lifetime odometer are redrawn in whichever unit you pick, not recalculated.
- Distance and altitude follow the speed unit automatically.
An app that rewrites stored data on every switch can pick up rounding error. One internal unit, converted at display time, can’t.
What happens to my speed alarm when I switch? #
Speedometer GPS stores the alarm limit as a real speed, so switching units keeps the same threshold and shows it converted and rounded. A 50 km/h alarm becomes 31 mph. That’s rarely a number you want, so after switching:
- Open Settings and look at the speed limit alarm.
- Tap one of the presets for the new unit.
- Fine-tune with − and +, which move in steps of 5 in km/h and mph and 1 in knots.
| Unit | Presets |
|---|---|
| km/h | 30, 50, 60, 80, 100, 120 |
| mph | 25, 35, 45, 55, 65, 75 |
| knots | 5, 8, 10, 20, 30 |
The mistake to avoid is picking a number before checking the unit. Choose 55 while the app is still in km/h and the alarm goes off at 34 mph, constantly. Switch the unit first, then pick the limit, and do it before you set off. How the alarm behaves once it’s set is in our guide to speed alert apps.
Why don’t my mph and km/h readings seem to match? #
The maths is fixed, so when numbers disagree, one of these is going on:
- Rounding. 50.0 km/h is 31.07 mph and shows as 31.1. Convert 31.1 back and you get 50.05 km/h. Neither is wrong.
- You’re comparing against the car. Most car speedometers are set to read a little high, so a dash showing 100 km/h while the app shows 96 isn’t a unit problem. GPS vs car speedometer explains the gap. Dual-scale dials add a second trap: at a glance it’s easy to read the inner km/h ring as mph.
- Different moments. The app averages the last five one-second readings, so it trails a changing speed by a second or two. Read mph, switch units, read km/h, and you’ve compared two different speeds.
- Live speed vs average. Average speed counts only time spent moving, so converting it and comparing with a live reading won’t line up.
- A lazy factor. Using 1.6 instead of 1.609 puts you almost 1 km/h out at 100 mph. Harmless on the road, visible if you’re checking an app’s maths.
- Knots mixed up with mph. A knot is 15% more than a mph. A boat doing 20 knots is doing 23 mph.
- The wrong unit is selected. This is the one that causes real trouble: a phone left in km/h on a US road, or in mph in a rental abroad. “60” means 60 mph or about 37 mph depending on the label next to it. Check the label, not just the digits.
Do saved trips change when I switch? #
No. In Speedometer GPS, trips are stored in metric and redrawn in whichever unit you pick, along with the lifetime odometer on the Trips screen. A 100 km trip appears as 62.1 miles in mph mode and switches back when you return to km/h. Switching mid-trip doesn’t reset anything either; only the display changes.
GPX exports don’t carry a unit at all. They store positions and timestamps, so the app you import one into works out distance in its own settings. If a route seems to grow by 60% after an import, one side is showing miles and the other kilometres.
Which unit should you record trips in? #
Whichever you’ll use without converting. If you log fuel or mileage in miles, record in mph mode; if your maps and road signs are metric, use km/h. Converting afterwards is exact, but switching back and forth makes it easy to write down the wrong number. For boats, knots and nautical miles are the natural pair; see knots vs mph for boat speed.
Before a cross-border drive #
- At your last stop before the border, switch the app to the local unit so it matches the signs.
- Re-pick the speed alarm from the new unit’s presets.
- Mount the phone and let it get a fix. Speedometer GPS works without mobile data, so roaming doesn’t matter.
- Learn the two or three limits you’ll see most. In Canada, 100 km/h is 62 mph and 50 km/h is 31 mph.
Conversion charts, mental-math shortcuts and what else changes at the US-Canada and US-Mexico borders are in our mph to km/h conversion guide.
Frequently asked questions #
Does changing units affect GPS speed accuracy? #
No. The speed is measured in metres per second and converted for display, so km/h, mph and knots are all the same measurement. Accuracy depends on the satellite signal, not the unit.
Does switching units reset my trip? #
Not in Speedometer GPS. The trip keeps counting the same distance and speed, and only the display changes. Saved trips also redisplay in your current unit, so you can review an old trip in miles even if you drove it in kilometres.
Why does my app show feet instead of miles? #
For short distances Speedometer GPS shows feet in mph mode, or metres in km/h and knots mode, then switches to miles, kilometres or nautical miles once the distance grows past 0.1 mile, 1 km or 0.1 nautical mile. It keeps the first part of a trip readable.
What does my speed alarm show after I switch units? #
The same threshold, converted and rounded, so 100 km/h shows as 62 mph. The alarm still fires at the same true speed. It’s usually easier to tap one of the new unit’s presets than to live with a converted odd number.
How many km/h is one knot? #
Exactly 1.852 km/h, because a nautical mile is defined as 1,852 metres. That’s about 1.151 mph. In knots mode, Speedometer GPS shows distance in nautical miles.