Are Vehicle Odometers Accurate?

The in basket: Bruce Landram of Belfair e-mailed an intriguing notion. “An automotive engineer friend of mine who works for GM in Detroit,” Bruce said, “explained to me that the law allows a speedometer to be +/- 10 percent from exact speed. The odometer is pegged to the speedometer.
“He explained that car companies intentionally set the speedometers 10 percent faster than the car is actually traveling. This way, the care gets to its warranty expiration mileage 10 percent sooner than if it were dead on accurate.

The in basket: Bruce Landram of Belfair e-mailed an intriguing notion after reading the Feb. 6 Road Warrior about the disparity between what a lighted speed readout on Mile Hill in Port Orchard says and what the speedometer on the car being paced says.
He’d had the same experience, he said, with three different cars showing he was traveling three miles per hour faster than the sign said he was.
“An automotive engineer friend of mine who works for GM in Detroit,” Bruce said, “explained to me that the law allows a speedometer to be +/- 10 percent from exact speed. The odometer is pegged to the speedometer.
“He explained that car companies intentionally set the speedometers 10 percent faster than the car is actually traveling. This way, the care gets to its warranty expiration mileage 10 percent sooner than if it were dead on accurate.
For example, 38 mph on the (speedometer), when the car is actually traveling 35 mph, is perfectly legal, but will get your car to show 36,001 miles (and out of
warranty) when the car has actually traveled only 33,000 miles,” Bruce said.
“It was actually the Japanese who started this in the late ’70’s. GM
followed suit three years later.”
“While the technology easily allows for an exact setting, the car companies
save tens of millions of dollars in not having to pay for work done (between
36,001 and 39,000 actual miles) that would have otherwise have had to be
covered under warranty,” Bruce asserts.
The out basket: I didn’t find this on a couple of urban legend sites I checked, Christine Fox who handles equipment issues for the State Patrol, said she can find neither state nor federal law setting the +/- 10 allowance.
But the claim rings a bell with me.
For years, I have been using those highway-side speedometer checks to see how accurate my odometer is. I routinely have found that no matter what vehicle I was driving, it would gain about a tenth of a mile every three miles. That comes to an extra mile every 30, and extra 10 miles every 300 and so on. In 30,000 miles, that’s an extra 1,000 miles, less than the 10 percent Bruce hypothesizes but substantial.
Never having needed warranty work on a car, the conclusion he reaches hadn’t occurred to me, but it pencils out.
This seems a perfect subject for the the new Kitsap Sun blog that features Road Warrior issues, drawing on reader knowledge. You can find it on line at kitsapsun.com. If any of you have some dope on the truth or falsehood of this claim, tell us what you know.