It depends on a lot of factors, but with GPS there will always be lag and incorrectness. For inner-city driving, the GPS positioning might be really accurate bu

It depends on a lot of factors, but with GPS there will always be lag and incorrectness. For inner-city driving, the GPS positioning might be really accurate but I doubt the speed calculation would be accurate, due to the constant acceleration, deceleration, and turning. The car's accelerometer will always be more correct. Plus, my Galaxy S Vibrant has the worst GPS ever released, so I'm very frustrated with not having reliable navigation.