I have a newer one and have used it quite a bit and if you take the time to calibrate and set it up correctly it can be very accurate. But some vehicles you will have a hard time setting it up because it will get a "dirty signal" from the power source. It doesn't take readings from the engine thru the cig lighter, it uses the voltage reading from the cig lighter (or other 12v source if it's hardwired) to read the rpm's of the engine. It uses that reading along with the reading from the gyros, and gives your results based on the distance v. time formula. It's kind of weird how it works, but it's a good tool to have if you tweak you car a lot and want to see how different changes affect your cars performance.

One thing I HATE about it is the suction cup does not stay put and will fall out of your windshield causing it to "thunk" on the dashboard which can't be good for those sensitive gyros on the inside. It comes with a permenant mount kit but I can't bring myself to drill/screw into my interior. Well..hope that was helpful.