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:
- the speedometer
- the ECU
- a separate GPS
- 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 speedometer consistently shows a speed about 5% higher than reported by the ECU
- methods 3 (GPS) and 4 (recording distance) are most accurate and are within about 0.5% of each other
- true speed is about 2-3% lower than what the speedometer shows
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).
On one particular trip on Saturday June 21st, 2014 I measured speed:
- using a GPS (a fairly old Garmin Legend, set to log track points every 5 seconds)
- from the ECU, using a laptop running Linux,
connected to it via bluetooth, using an OBD2 bluetooth
adapter (see this page
for more info on getting this to work)
- from the speedometer (simply by enabling cruise control on certain stretches and noting
- by recording distance markers ("hectometer paaltjes") along the highway spaced 100 meters
apart using a Canon EOS1100D recording at 25 frames/second (operated by a passenger of course :-))
(more to follow)
Last modified: Fri Jun 27 21:59:21 CEST 2014