Measuring the speed of your car

If your speedometer says you're driving 105 km/hour (about 65 mph), what are you actually driving? Which is more accurate, your Tomtom GPS or your speedometer? What about the values reported by your car's ECU (Engine Control Unit)?

Search the web for this and you find many pages with these questions and quite a few different answers (I'd be interested to know of any pages reporting measurements like the ones below).

I set out to answer these questions by measuring my car's speed using four different methods:

  1. the speedometer
  2. the ECU
  3. a separate GPS
  4. by recording distance markers on the highway using a digital camera

If you don't want to read the whole page, here are the main conclusions:

The bottom line is that if you drive by your speedometer, you're less likely to get a speeding ticket when you accidentally go a few miles over. This seems consistent with everyone's experience: a car manufacturer would be liable if the speedometer would underreport speed, so a safety margin is added. This margin is also needed because the ECU doesn't know the exact circumference of the wheels (from which the speed is calculated).

The data

On one particular trip on Saturday June 21st, 2014 I measured speed:

(more to follow) Last modified: Fri Jun 27 21:59:21 CEST 2014