My speedometer is lying!

When driving in my car I regularly look at my speedometer, just to make sure I’m not driving too fast. But how usefull is this if the speed I’m reading on the speedometer is wrong. I knew there was an offset, but I never checked how much the offset was.

After “extensive” research my conclusion is: My speedometer is lying!

Recently a law has changed in The Netherlands, due to this new law the maximum speed is increased from 120 KM/h to 130 KM/h. This raised some questions to a lot of drivers, usually about speeding tickets. Since the police is using a correction of 3% this means you can drive 133,9 KM/h without getting a speed ticket, right?

Well, it depends on where you’re looking for the speed you driving. My speedometer has an offset of roughly 10%, which is a lot IMO.

The suspect

Type: Volkswagen Scirocco 2009 (and yes, it’s that green. Green IT people!).

Engine :1.4TSI, 160bhp

Tiresize : 235/40R18

Getting the numbers

To determine the offset I’ve used three sources. First the speedometer in front of me, this is my main source for getting information about the speed I’m travelling. Second is the speed the car thinks I’m driving, in a Volkswagen (and other cars from the VAG group) this can be read from the CAN-bus.

I can read the speed, according to the CAN–bus, by pressing the AC button and the “blow in my face” button at the same time and selecting 19.0 (on the left display). The CAN-bus speed is displayed on the right display. If you want MPH, select 19.1 (there are more things you can read btw).

The third source is the GPS, which is the most accurate. The car is equipped with a RNS510 navigation system (atleast until now, they get stolen a lot unfortunately). By pressing the Setup button long enough, the Test mode is displayed (if this is enabled in your device). There you will find a menu ‘Navigation’ containg a submenu ‘Matched Position’. This will display the speed according to GPS.

The facts

I’ve been driving around at different speeds, based on my main source the speedometer, and wrote down the speed that was given by the other sources (CAN-bus and GPS). I’ve placed all numbers down in an Excel worksheet and created charts.

Based on the numbers, the facts, I can conclude a few things. First, the GPS has a greater offset then the speed displayed by the CAN-bus. Based on research on the (very reliable) internet, I concluded that GPS is the most accurate. Which makes sense since GPS calculates the travelled distance of a certain time without the need of knowing what tiresize you’re using (later more on that).

Second, the offset between the speedometer and the GPS is a whopping 10%!

What is causing this offset?

The offset is caused by the car manufacturer.The speed displayed on the speedometer is based on pulse signals from the wheels and an algorithm. Each time the wheel makes a complete circle (or a portion), a pulse is given to the car electronics. The car electronics “knows” how many (centi)meters one circle is and thus the speed. The problem is that the size of tires change. Not only because people use other sizes (16” or 28”) or brand of tires, the tires will wear off and get smaller. Even the air pressure can change the size of a tire, and thus the distance elapsed.

Car manufactures don’t like to be sued because the speed displayed in lower then you’re really driving. This means you, as a honest citizen, are driving the allowed speed and still receive a speeding ticket due to a wrong speedometer. That’s why the speedometer has an offset, just to make sure that whatever tiresize you’re using (etc.) the speedometer is “safe”.

Conclusion

If you want to “live on the edge”, and drive as fast as possible without getting a ticket, check the GPS speed. Or, get your speedometer calibrated by a professional, just like the police. And the next time you get a speeding ticket, don’t complain that you where traveling “just a few km/h to fast”, it was probably a lot more on your speedometer

Ingmar Verheij