Speedometer vs. trip computer vs. GPS
 

When I originally bought the car, I checked the speedo error versus a GPS unit, and found out that for example speedo-90kph was actually 83kph and speedo-60kph was actually 54kph, and so on. Well, that was fine and all as I new speedos always show a bit extra and it's easy to position the needle on the "tens" rather than increments.

Today, I noticed something odd. I drove on a motorway, set the cruise control to 90kph, and then reset the trip computer. The average speed was 83kph, and stayed 83kph for the several kilometres I drove at indicated 90kph. I then switched to a smaller road, set cruise to 60kph, reset trip, and got an average of 54kph that also stayed for as long as I kept at indicated 60kph.

So, the trip computer seems to get the accurate speed information from somewhere, but where? My car has the satnav and thus it has a GPS, but I can't imagine Jaguar would've made the satnav-equipped cars use GPS speed in trip computer...

It's obviously not possible/desirable to have the speedometer use GPS data (tunnels, errors, stuff), but can the speedometer be calibrated to more closely follow the true speed?

EDIT:
Just after posting, heard that the car ECU knows the "real" speed (if on stock tires, outer diameter that is), but the speedometer show's incorrect values deliberately.

You can see the difference between the ECU speed and the speedometer indicated speed by entering the diagnostic mode via the trip computer button.

The instructions are here: Jaguar X Type Instrument Cluster Diagnostic codes


One of the options gives you speed in km/h - the description is "Displays speed input value after compensation in tenths of km/h, no decimal point shown. Speedometer will indicate present speed. Displays '----' if message is not received or if received data is invalid for two seconds or more."

On my car, at 80km/h on the ECU, the speedo needle is reading about 75.

Oh, and I'm not aware of anyway of recalibrating it!

Cheers,

Rob

Right, I have been to the diagnostic mode but didn't think of it now. Good idea.

Here it's illegal for a car speedometer to display less than the actual speed, perhaps that is why all speedometers are "a bit optimistic". Perhaps it's best to be on the safe side, and if I do indicated 90kph on a 80kph road I'm not going fast enough to trigger any cameras, and I seem to be quite perfectly in pace with the rest of traffic.

Sorry, I got it the wrong way around. When indicating 80, the ECU speed is 75!

Regards,

Rob

As far as i know, all speedos are calibrated to read slightly more than the actual speed to prevent manufacturers being sued by owners if caught speeding.

That's not entirely true. Some speedos show lower speed than the actual (some versions of the Nissan Murano did that for instance).

As any measurement devices, speedos have acceptable errors (+- a few %s). For that reason many countries have warnings before they can start ticketing "speeding" drivers. Commonly fines applied from 10-20 km/h above the actual speed limit.

You will notice that most of the posters are located in the EU.

There is a EU standard addressing speedometer error. It allows zero percent under reading but X percent over reading. Thus, manufacturers bias the calibration in light of the standard.

Thanks for pointing that the majority of the posters are from Europe :)

I was talking about early Russian versions (not sure about nowadays though) of the Murano and some other cars that had this "speado feature". There are a lot of variables in this equation that could affect the speedo accuracy in one way or another.