Rollback Your Car Odometer with Motor MythBusters!09/09/2021
450,000 cars sold each year are victims of odometer fraud—taking a car odometer and rolling back the displayed mileage to increase the perceived value of the car. Odometer rollback is a federal offense, and modern cars make it very difficult to do, but according to pop culture, rolling back the mileage is as simple as driving in reverse. Yeah, the Motor MythBusters don’t believe that for a second.
Related: Odometer fraud is illegal, but streaming Motor MythBusters—only on the MotorTrend App—isn’t! Better yet, it’s fun and educational and there’s thousands of hours of other great automotive shows to enjoy, too! Your free trial is only a click away.
In the 1986 movie Ferris Bueller’s Day Off, the titular role—played by Matthew Broderick—convinces his best friend to take his dad’s pristine 1961 Ferrari GTO California Spyder on a day-long joyride while they ditch school. Obviously, daddy would notice the increase in mileage, but Ferris assures his friend they can simply put the Ferrari on jackstands and let it run in reverse for a while. You’ll have to watch the movie (and this episode of Motor MythBusters) to find out if that worked.
How Does a Car Odometer Work?
Back in ye olden days, when a car odometer was an analog affair, the dial was turned by a cable connecting the odometer and transmission. That actually wasn’t that long ago; most car manufacturers started phasing out mechanical odometers in favor of digital versions in the early 2000s.
Rolling back to how mechanical odometers actually functioned, the transmission output shaft of a car is equipped with a specially designed spline gear that drives the speedometer gear. Yes, we’re talking about recorded distance, not speed, but it’s all the same assembly.
Fun Fact: Two of the last cars sold in the United States with mechanical odometers were the 2003 Pontiac Grand Prix and the 2005 Ford Crown Victoria and Mercury Grand Marquis (Canadian-built models).
The speedometer gear spins a cable that is connected to a set of gears that turn a numbered dial that displays mileage using the mathematical magic of gear reduction. The rotating speed and diameter of the transmission output shaft spline gear, the diameter of the speedometer gear, and diameter of the car’s drive tires all affect the odometer’s output. This is why changing the diameter of a car’s drive tires without changing the speedometer gear can affect the displayed speed and recorded miles.
It’s easy to think that because there is a direct mechanical connection from the odometer gear set to the transmission output shaft, turning the transmission the opposite way would roll back the miles. Indeed that was the case until around the 1960s, when auto manufacturers started making more sophisticated odometers capable of recording distance traveled in both forward and reverse with ratcheting and free-wheeling gears like you find on a bicycle.
Modern digital odometers replace the clockwork and cables with magnetic sensors that record the number of times the transmission output shaft rotates. That information goes to the car’s onboard computer, and after some electrons shoot through processors and wires, you’ve got all the information you need to operate your car.
How to Spot Odometer Fraud
Mechanical or digital, odometers can be changed if one tries hard enough. But the Motor MythBusters want to arm you with the knowledge of what to look for when a used car seems too good to be true. Sometimes, with mechanical odometers, it’s as simple as checking to see if the numbers line up properly. Digital odometers that can be changed with professional-grade diagnostic tools and are more tricky to spot odometer fraud.
With either case, think of all the contact points you have inside a car: the driver’s seat, steering wheel, pedals, sun visors, various switches and buttons—unless those are replaced or refurbished when the miles get rolled back, they will all display signs of wear commensurate with the actual mileage of the car. People who are willing to commit odometer fraud to make easy money are probably not likely to put lots of effort into covering up these small but telltale signs of use.
The traction pad on the brake pedal in a car with less than 20,000 miles shouldn’t be falling off or showing the metal underneath. Same for other high-wear areas like the seat bolsters or window switches. The steering wheel shouldn’t be deteriorated or feel worn. Even fresh tires on a low-mileage car can be a clue that the odometer has been tampered with.
A low-mileage car shouldn’t have many interior blemishes that can be associated with regular use. This is the opposite of the classic colloquialism, “never look a gift horse in the mouth.” We say, “always look a used car in the pedal box,” and the Motor MythBusters agree.
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