Helpful Hints from joeeze: Can you trust your speedometer?

Yesterday's Wall Street Journal "Me & My Car" column by Jonathan Welsh addressed precisely that question.

Here's the Q&A.

Q. At higher speeds, the number displayed on my Global Positioning System is 3 to 5 miles per hour less than that shown on my car's speedometer. Which one should I believe?

A. Trust your GPS. Speedometers are notoriously inaccurate, though typically their readings overstate a car's speed, which is better than underestimating them. The same is true for motorcycles. Your speedometer's degree of error is typical, though sometimes the difference can be greater than 5 mph.

Some car makers suggest this difference between actual speed and measured speed is no accident because it theoretically keeps highway speeds a bit lower.

Subtle differences in tire sizes can also throw speedometer readings off, but even the most basic, inexpensive GPS units tend to be accurate.

[via Jerry Young]