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.

4 thoughts on “Are Vehicle Odometers Accurate?

  1. Your column on speedometer accuracy fits my ’04 Buick Rondezvous to a T. My Buick’s speedo is off about 5% as measured with mileposts and to routes driven with the ’98 Dodge Caravan I previously owned. The error is in Buick’s favor, of course. I mentioned this at one time to a Buick service person and was told that wasn’t a commmon problem but to bring it in and they’d check it. I never folowed up, but will do it now.

  2. My experience is that the speedometer may be off, but the odometer is not. When I put slightly larger than stock tires on my Nissan pick up, I wanted to find out how far off the speedometer was going to be. I calculated what the error should be based on the increased tire diameter, as the speedometer should now be indicating a slower speed than what I was actually driving at. The change in tire diameter was somewhere around 10%, but when I checked the odometer and speedometer on a 10 mile marked section of I-5, I disovered that the speedometer now read exactly the right speed, but that the odometer was off by the calculated amount. (I actually drive farther than what the odometer says I drive.) So in my case, the odometer had been correct, but the speedometer was indicating a higher speed than reality in its factory state.

  3. I checked our 2000 Acura 3.2TL, when new, with my GPS on a 750 mile trip. The odometer was 4.56% in favor long, which equates to 4.56% slow on speed. I employ this knowledge to nudge the speed upward and get slightly better trip times. GPS will accumulate miles traveled as well as show instantaneous actual speed of advance.

  4. If the readings are higher than actual speed and distance traveled, wouldn’t that make it extremly difficult for the auto makers to meet the mpg standards EPA sets? E.G. If my odometer says I traveled 20 miles and I used 1 gallon of gasoline that would be 20 mpg. If the odometer was 10 percent high I would have went 18.18 actual miles and got 18.18 mpg.

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