A straight, horizontal road...
Assuming a fixed speed on horizontal road, a change in altitude would increase the speed reading (think Pythagoras and triangles).
Cheers,
My Garmin always gives me the altitude - I think the satellite info is 3-D (it must be, surely?), so it should calculate the true speed whatever the gradient. Anyway, even if that is not the case, you would need a fairly steep hill to make an appreciable difference. My trig suggests a 10% grade (1 in 10) would give an error of 0.5% - most long straight roads are a lot flatter than this.
Cheers
Fjemog
