We leased a new Volkswagen Golf recently. On the way back from Buffalo, my wife noticed a discrepancy between the speed on the speedometer and speed on the GPS readout. At home, we went on the net and found that this is well known about VW. We've been told that there is definitely a correspondence between the speedometer and odometer. In a leased car, going over the lease limits could be quite costly. Should we be worried that the odometer is also especially inaccurate?
Concerned that your new car is faster than the speedometer says? Speedometers are designed to err slightly on the fast side, but your odometer should still be accurate, says Volkswagen.
"Speedometers are designed to never show speeds lower than actual speed." Thomas Tetzlaff, Volkswagen Canada's media relations manager, wrote in an email. "In order to do this... they will necessarily always show speeds slightly in excess of actual speeds."
While speedometers are calibrated to fudge the numbers a little, odometers are designed to reflect accurate mileage, Tetzlaff says.
The odometer shows the distance your vehicle has travelled, while the speedometer show how fast your vehicle is going.
"The accuracy of the speedometer in most vehicles, including Volkswagens, is generally within a few percentage points of actual speed," Tetzlaff says. "Odometer readings are designed to be quite accurate."
Why is there a built-in factor of error for speedometers? Speed is a measure of distance over time. That's what the GPS is showing. But speedometers figure this out by measuring how fast the wheels are spinning using the speed sensor in the transmission.
If the diameter of the wheels changes – and it does change, depending on tire size, pressure and wear – the accuracy of the speedometer gets thrown off.
Car and Driver wrote about this more than a decade ago, and their explanation still stands.
"Normal wear and underinflation reduce the diameter of the tire, causing it to spin faster and produce an artificially high reading," writes Frank Marcus. "Overinflation or oversize tires slow down the speedometer."
The difference between actual speed and the speed dial tends to get noticed more with German cars because their speedometers are designed to never report a speed lower than actual speed. European law (ECE-R39) says speedometers cannot show speeds less than the actual speed, and they must never show more than the 110 per cent of actual speed plus 4 km/h. So, under those rules, a car could be moving at 100 km/h, but the speedometer could legally display as high as 114 km/h.
But, generally, most speedometers exaggerate speeds, says Wall Street Journal columnist Jonathan Welsh. "Industry officials have said speedometers are set 'optimistically' to give drivers an inflated sense of speed (and help them avoid tickets)." Welsh says. "GPS devices record distance over time without the effects of mechanical variables and thus tend to be more accurate."
Anything that changes wheel diameter – such as putting on bigger tires – will affect the accuracy the odometer.
The odometer, which directly reflects what the sensors are recording, doesn't have the same built-in inaccuracy as the speedometer. If your speedometer is off, it doesn't necessarily follow that the odometer is inaccurate too.
A difference between the speed on your GPS and the speed displayed on your car's speedometer is normal, and it doesn't mean that your odometer is out of whack.
Bottom line: Off the lot with the manufacturer-installed tires (like your leased VW Golf), there's a good chance that your speedometer is inaccurate, on purpose. Often cars displays a speed faster than your car is really going. This is pronounced in European cars which calibrate speedometers so they never show a speed slower than you're traveling, regardless of tire inflation and tire size. The odometer leaves the factory calibrated to display accurate mileage, manufacturers say. That said, make any significant changes to tire size, and both the speedometer and the odometer will be inaccurate.
Correction: Transport Canada says there are no federal regulations in North America regarding the accuracy of speedometers. Because speedometers and odometers are calibrated to the diameter of the original tires, Transport Canada spokesperson Maryse Durette says, "if the owner departs significantly from the manufacturer's original specifications at the time of purchasing replacement tires, the speedometer will no longer be in calibration."
In an earlier version of this story, Motor Trend was incorrectly credited with the first link, but in fact it should have been Car and Driver. The error has been corrected.
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