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On a recent 800 mile trip on US interstates and in Canada, I took the opportunity to check the accuracy of the odometer and speedometer on my 2001 XC. My method was to use the mile (km in Canada) markers, with comparisons over typically 10 miles to average out minor errors in the marker positions, and, for the speed tests, to use a stop watch over several miles with the selected indicated speed maintained precisely with the cruise control.
Errors were further averaged out by completing 8 speed tests and 10 distance tests. After returning home, I calculated the errors in both miles (and mi/h) and km (and km/h), and produced a percentage calibration error (errors as a percentage of the TRUE speed or distance). The summary results are:
SPEED: the speedometer reads consistently 6.3% high - measured at various speeds between 60 and 75 mi/h (about 96 - 121 km/h).
Examples of true speeds with indicated speeds in ( ): 60 mi/h (63.8); 65 mi/h (68.9); 70 mi/h (74.4); 75 mi/h (79.7); 100 km/h (106.3); 120 km/h (127.6); 130 km/h (138.2). Put another way, if I think that I am cruising along at 75 mi/h (mph) as indicated on the speedometer, with a legal speed limit of 70 mi/h, I am not actually exceeding the speed limit, bearing in mind that the speedo pointer thickness is about one mi/h! And the cars overtaking me at what I think is 80 mi/h are only about 5 mi/h above the limit and would probably not be pulled over in a speed trap.
DISTANCE (odometer and trip readings): the indicated distances are much more accurate than speed. The odometer reads high with an error of +0.7% compared with true distance. This is probably not going to be significant for most needs except that I will be actually forfeiting about 1100 km (700 miles) from my extended warranty!
The speed error does raise some issues for me, for which I would be interested in feedback:
1. Since the electronic speed computation has a reasonably accurate input of distance, and assuming that the internal computer clock is accurate, the 6.3% error must presumably be deliberately programmed in by Volvo. If not, how would it come about?
2. I would rather know more precisely what speed I am doing rather than Volvo providing me with a hidden cushion against speeding tickets.
3. What Volvo seems to gain is a customer who is more impressed by the effortless high cruising speed than is actually the case, and an optimistic sense of 0-60 acceleration times.
4. Being electronic, the error is perhaps consistent from car to car? I wonder if the dealer can do a software adjustment?
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