Written by Muneeb Rana · Last updated 28 October 2026
Quick answer: A phone can't read engine RPM directly. A tachometer app gets it from the car's computer through an OBD-II adapter (most accurate), from engine sound via the microphone (rough), or by calculating it from GPS speed and gear ratios (an estimate). A GPS speedometer shows road speed, not RPM.
TL;DR
- OBD-II adapters read RPM straight from the engine computer (PID 0x0C). This is the most reliable phone method.
- Sound-based apps convert firing frequency into RPM and need your cylinder count.
- GPS can only estimate RPM if you know your gear ratios, final drive and tyre size.
- Speedometer PRO shows speed, not RPM, and doesn't connect to OBD.
Three ways a tachometer app gets RPM
| Method | What you need | Accuracy | Works on |
|---|---|---|---|
| OBD-II adapter | Bluetooth or Wi-Fi OBD dongle + app | High: reads the engine computer | Most cars sold in the US since 1996 |
| Engine sound (microphone) | Phone near engine, cylinder count | Rough, struggles with noise | Any engine, including mowers and bikes |
| GPS + gear math | Speed, gear ratio, final drive, tyre size | Estimate, assumes no clutch slip | Manual gearbox in a known gear |
| Optical / strobe | Reflective tape + camera or laser | Good for exposed shafts | Fans, drills, propellers |
The US EPA notes that OBD systems have been required on US cars and light trucks since model year 1996. OBD-II defines a standard data item for engine speed. PID 0x0C returns two bytes, A and B, and RPM = (256 × A + B) ÷ 4.
How sound-based tachometer apps work
A four-stroke engine fires each cylinder once every two revolutions. So the firing frequency is:
Firing frequency (Hz) = RPM × cylinders ÷ 120
Worked example: a 4-cylinder engine at 3,000 RPM gives 3,000 × 4 ÷ 120 = 100 Hz. A sound app hears a strong 100 Hz tone and works backwards to 3,000 RPM. If you tell it the wrong cylinder count, say 6 instead of 4, it would report 2,000 RPM. That's why these apps always ask.
Wind, road and exhaust noise blur the tone, so these apps work best at idle or in a garage. They're handy for small engines with no tachometer, such as mowers, generators and outboard motors.
Estimating RPM from GPS speed
If your car has a manual gearbox and you know the ratios, you can estimate RPM from road speed:
RPM ≈ mph × gear ratio × final drive ratio × 336 ÷ tyre diameter (inches)
Worked example: 60 mph in a 1.00 gear with a 3.73 final drive on 28-inch tyres: 60 × 1.00 × 3.73 × 336 ÷ 28 = 2,686 RPM.
| Speed (mph) | 4th gear 1.00, 3.73 final, 28" tyre |
|---|---|
| 30 | 1,343 RPM |
| 50 | 2,238 RPM |
| 60 | 2,686 RPM |
| 70 | 3,133 RPM |
This assumes no wheel or clutch slip, and with an automatic it ignores torque-converter slip. Use GPS speed, not the dash, because the dash usually over-reads a little.
Choosing a tachometer phone app: checklist
- Need exact RPM? Buy an OBD-II adapter that lists your phone (iPhone needs Bluetooth LE or Wi-Fi adapters).
- Small engine, no OBD? A sound-based app plus the correct cylinder and stroke settings.
- Just curious about cruise RPM? Use the GPS formula above.
- Check the app's permissions: microphone for sound apps, Bluetooth for OBD, location for GPS.
- Never read RPM on the phone while driving. Mount it and glance, or log the drive and review it later.
Speed vs RPM: what Speedometer PRO does
Speedometer PRO is a GPS speedometer. It shows road speed in mph or km/h, max and average speed, distance and a HUD view. It doesn't read RPM and doesn't connect to OBD. If you want speed alongside a tachometer app, run the tachometer app on an OBD dongle and use your car's dash or a second device for GPS speed.
Related: How GPS speedometers work · How to check speedometer accuracy · HUD speedometer guide · GPS speedometer guide · Android Auto Speedometer and CarPlay Speedometer Apps · Video Speedometer App: Recording Speed on Video · All speedometer guides
Get Speedometer PRO on Google Play · Download on the App Store · Set it up before you move; never handle your phone while driving.
Sources
- Wikipedia: OBD-II PIDs (PID 0x0C engine speed): en.wikipedia.org (accessed 28 October 2026)
- US EPA: On-Board Diagnostics (OBD): epa.gov (accessed 28 October 2026)
- GPS Accuracy (GPS.gov): gps.gov (accessed 28 October 2026)
- Tire code (Wikipedia): en.wikipedia.org (accessed 28 October 2026)
- Speedometer PRO・MPH Tracker (Google Play listing): play.google.com (accessed 28 October 2026)
- Speedometer PRO・MPH Tracker (App Store listing): apps.apple.com (accessed 28 October 2026)
FAQ
Can my phone be a tachometer?
Yes, with an OBD-II adapter (most accurate), a sound-based app (rough) or a GPS-and-gear calculation (an estimate).
What is the best tachometer app for iPhone?
Pick by method. For cars, an OBD app paired with a Bluetooth LE or Wi-Fi adapter. For small engines, a sound-based app.
Can GPS measure RPM?
Not directly. GPS gives speed. You can work out RPM from it if you know your gear ratio, final drive and tyre size.
How do sound tachometer apps calculate RPM?
They detect the firing frequency. For a four-stroke engine, RPM = frequency × 120 ÷ cylinders.
Does Speedometer PRO show RPM?
No. It shows GPS speed, distance and trip data.