If your car uses a manual gearbox, you can save fuel and keep exhaust emissions to a minimum if you don’t overrev your engine. A shift light mounted on the dash can tell you exactly when it’s time to upshift. The best part? You can easily add one to almost any car via the OBD2 port.

OBD2 Diagnostic Module

Back in the day, a rev counter was often seen as more important than the speedometer. If you want to drive more efficiently — or just a bit sportier — keeping an eye on your engine’s RPMs is key for knowing when to change gear. Many economy cars don’t come with a tachometer, and that’s where a shift light can really come in handy. It’s basically a light that flashes when it’s time to shift up. These gadgets used to be mocked as gimmicks, but they might be making a comeback. Most modern cars are now so quiet (especially diesels) that it’s hard to tell how hard the engine’s working just by listening.

If your car doesn’t have a rev counter or shift light and you think it might be u...