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. "Overinflation or oversize tires slow down the speedometer." "Normal wear and underinflation reduce the diameter of the tire, causing it to spin faster and produce an artificially high reading," writes Frank Marcus. ![]() 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.Ĭar and Driver wrote about this more than a decade ago, and their explanation still stands. But speedometers figure this out by measuring how fast the wheels are spinning using the speed sensor in the transmission. Why is there a built-in factor of error for speedometers? Speed is a measure of distance over time. "Odometer readings are designed to be quite accurate." "The accuracy of the speedometer in most vehicles, including Volkswagens, is generally within a few percentage points of actual speed," Tetzlaff says. The odometer shows the distance your vehicle has travelled, while the speedometer show how fast your vehicle is going. While speedometers are calibrated to fudge the numbers a little, odometers are designed to reflect accurate mileage, Tetzlaff says. they will necessarily always show speeds slightly in excess of actual speeds." ![]() "Speedometers are designed to never show speeds lower than actual speed." Thomas Tetzlaff, Volkswagen Canada's media relations manager, wrote in an email.
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