Automotive Trivia
A Speedometer's
Inner Workings
Ever wonder how your speedometer calculates just how fast you're going? If you think it's measuring your spinning tires, you're almost correct. Instead, it's connected to a little gear on your transmission by a cable that reads your revolutions per minute (RPMs). That's right — a speedometer is just a tachometer in sheep's clothing. Of course, the important difference between a tachometer and a speedometer is that one just displays your RPMs and one is calibrated to translate those rotations into an approximate speed. Many modern cars, trucks and SUVs use electronic speedometers instead of the mechanical cable type, but the results are still the same.
Most speedometers are intentionally calibrated to display a higher reading than your true speed. In other words, if you're cruising down the freeway at 65 mph, chances are you're really going about 60. This over calibration is intended to keep drivers from exceeding the speed limits. Before you start revving up your engine, remember this: any change in tire size or inflation can yield faulting readings that counter the over calibration. You're best bet is to play it safe and forget all about the 5 mph buffer.