If you've ever modified a car with smaller or larger tires or, perhaps, a different final-drive gear, you've immediately screwed up your speedometer. This is, of course, annoying, but for modern cars ...

Your car once had a rock-steady speedometer needle, but now it’s wandering a bit — or a lot. Maybe it’s not indicating any speed or it’s showing 60 mph and you know that can’t be right. “An easy way ...