The European standard for speedometer accuracy is very strict, but apparently the US standard is not. Here is an excerpt I found in a C&D magazine about this subject.
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So we sought out the rule book to find out just how much accuracy is mandated. In the U.S., manufacturers voluntarily follow the standard set by the Society of Automotive Engineers, J1226, which is pretty lax. To begin with, manufacturers are afforded the latitude to aim for within plus-or-minus two percent of absolute accuracy or to introduce bias to read high on a sliding scale of from minus-one to plus-three percent at low speeds to zero to plus-four percent above 55 mph. And those percentages are not of actual speed but rather a percentage of the total speed range indicated on the dial. So the four-percent allowable range on an 85-mph speedometer is 3.4 mph, and the acceptable range on a 150-mph speedometer is 6.0 mph.
But wait, there's more. Driving in arctic or desert climates? You're allowed another plus-or-minus two percent near the extremes of 20-to-130-degrees Fahrenheit, and yet another plus-or-minus one percent if the gauge was ever exposed to minus-40 to plus-185 F. Alternator acting up? Take another plus-or-minus one percent if the operating voltage strays two volts above or below the normal rating. Tire error is excluded from the above, and odometer accuracy is more tightly controlled to plus-or-minus four percent of actual mileage.
The European regulation, ECE-R 39, is more concise, stating essentially that the speed indicated must never be lower than the true speed or higher by more than one-tenth of true speed plus four kilometers per hour (79.5 mph at a true 70). Never low.
