Speedometer vs. trip computer vs. GPS
#1
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.
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.
#2
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
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
#3
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.
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.
#6
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.
Last edited by Nardoswiss; 02-21-2013 at 11:46 AM.
#7
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.
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.
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.
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Nardoswiss (02-21-2013)
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#8
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.
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.
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