An odometer counts the miles or kilometers a vehicle has traveled. That number tells you how worn the brakes, suspension, and engine likely are. It also sets the resale value. Roll it back and you commit fraud in most places.

Mechanical odometers used a cable spun by the transmission. Gears turned numbered drums. The design was simple and easy to tamper with. Digital odometers store the reading in memory, which makes tampering harder but not impossible. Some used-car scams still erase or replace instrument clusters.

Odometer facts worth knowing

• US federal law requires odometer disclosure on title transfers.
• A car that shows 30,000 miles but has worn pedals and a sagging driver's seat may be lying.
• Service records and tire wear patterns help verify the reading.
• Some modern cars report mileage to manufacturers and insurers remotely.

An odometer is only as honest as the people who maintain it. A well-documented high-mileage car often beats a low-mileage car with no history.