[hipocrazy]

Peaked my interest, so i searched.

"GTech is an app made by irafaS that can measure speed (Km/h mph) but as well acceleration ( 0 - 100Km) of a moving vehicle and give direct results to the iphone screen.

This is made using iPhone's accelerometer"

The only app I've used (being that I'm on a touch with no accelerometer) is GPStoday which, by it's name alone, obviously use GPS instead of the accelerometer.
It measures my speed while driving pretty accurately.

Hope I helped a bit......