Blog · Speedometers
How to Read a Speedometer (mph, km/h and Metric Dials)
The needle, the two rings of numbers, the odometer and the tachometer — a plain-English guide to every number on the dashboard, including metric speedometers.
SpeedReflex · Updated · 6 min read
A speedometer shows your current speed: read the number the needle points to, using the scale for the unit you want.On cars sold in the US and UK the large outer numbers are miles per hour and the smaller inner numbers are kilometres per hour. Between the numbers, each small tick mark is an equal step — that’s all there is to it, but the details trip up new drivers and travellers alike.
How to read an analog speedometer
- Find the two labelled numbers the needle sits between — for example 40 and 60.
- Count the spaces between them. If there are four spaces between 40 and 60, each tick is 5 mph.
- Count ticks from the lower number. Two ticks past 40 is 50 mph.
- Read straight on. Looking at the needle from an angle — from the passenger seat, say — makes it appear to point at a slightly different number.
Most dials label every 10 or 20 mph with minor ticks every 2, 2.5 or 5. Many highlight common speed limits so you can find them at a glance, and the needle usually sits near the top of the dial at typical highway speeds.
Reading mph, km/h and metric speedometers
Dual-scale speedometers print both units. In the US and UK, mph is the main scale and km/h the secondary one; in Canada, Europe and most of the world it is the other way round. A metric speedometer reading is simply in kilometres per hour, and if you are driving abroad in a rental car the quickest mental conversion is km/h × 0.6 ≈ mph (more exactly × 0.62), ormph × 1.6 ≈ km/h.
| Metric reading | In mph |
|---|---|
| 30 km/h | 19 mph |
| 50 km/h | 31 mph |
| 60 km/h | 37 mph |
| 80 km/h | 50 mph |
| 100 km/h | 62 mph |
| 110 km/h | 68 mph |
| 120 km/h | 75 mph |
| 130 km/h | 81 mph |
| Reading in mph | In km/h |
|---|---|
| 20 mph | 32 km/h |
| 25 mph | 40 km/h |
| 30 mph | 48 km/h |
| 45 mph | 72 km/h |
| 55 mph | 89 km/h |
| 65 mph | 105 km/h |
| 70 mph | 113 km/h |
| 75 mph | 121 km/h |
Digital speedometers
A digital speedometer shows your speed as a number, sometimes alongside a graphic dial. Many let you switch between mph and km/h in the settings menu, and some project the number onto the windshield as a head-up display. Your phone can do the same: the freeonline speedometer is a digital GPS speedometer with a large readout, an analog-style gauge and a windshield HUD mode.
What does a speedometer measure — speed or velocity?
A speedometer measures speed, not velocity. Speed is how fast you are going; velocity is speed in a particular direction. Your speedometer shows 50 mph whether you are driving north or south, so it can’t be measuring velocity. In physics terms, the speedometer of an automobile reads instantaneous speed — your speed at this moment rather than your average over a trip.
How a speedometer works
- Mechanical (eddy-current) speedometers, used for most of the 20th century, are turned by a cable from the transmission. A spinning magnet drags an aluminium cup against a hairspring, and the faster it spins the further the needle moves.
- Electronic speedometers count pulses from a vehicle speed sensor or the ABS wheel sensors, and a computer drives the needle with a small motor or shows the number on a screen.
- GPS speedometers calculate speed from satellite signals, so they measure your movement over the ground directly and aren’t affected by tire size.
Because car speedometers count wheel turns, a change of tire size changes the reading — see thespeedometer calculator — and a fault anywhere in that chain can stop it altogether, as covered inspeedometer not working.
Odometer vs speedometer
They are easy to confuse because they sit together, but they measure different things. The speedometer shows how fast you are going now. The odometer shows the total distance the vehicle has ever travelled, and one or two resettabletrip meters show the distance since you last zeroed them. Both are driven by the same speed signal, which is why a failed speed sensor usually stops both.
Speedometer and tachometer
The other big dial on many dashboards is the tachometer. It shows engine speed in revolutions per minute, usually marked in thousands (“3” means 3,000 rpm). The red section is the redline: the maximum engine speed the manufacturer allows. Speedometers don’t have a redline — the red zone people mean is on the tachometer. Read together, the two tell you which gear suits the road: at a steady 60 mph in top gear most cars are turning roughly 1,500–2,500 rpm.
Speedometer symbols and lights
On many cars a small speedometer-and-arrow icon in the cluster is the cruise control indicator, lit (often green) when a cruise speed is set; a similar symbol with a different colour or a line may mean a speed limiter is active. Your owner’s manual lists every symbol for your model. If the speedometer’s own backlight is dim, check the dashboard brightness control before suspecting a fault.
Why speedometers read high — and go so high
Speedometers are designed to read slightly above your true speed, never below it; the car speed test shows by how much on your car. And if you have wondered why a family car’s dial runs to 140 or 160 mph, readwhy speedometers go so high.
FAQ