Halp detecting invalid values

Lets say i have this series of values, representing speeds:
50,60,60,120,61,65
The 120 is probably a read error, so i need to detect it as wrong and discard it. I know that because is 60, then 120, then 61.

But what if i have
50,60,60,120
And i discard 120 as out of range, but the next value is 121, and later 125. Then the 120 is probably ok. So i discard 120, but then i realize it is ok and i accept it again.

What algorithm can i use for that?

Oh oh right now I remember, you're the guy who wanted to use jewglemaps or whatever the fuck for your little thing. I hope you still aren't going to rely on jewgle. So now that I know what you're actually talking about, how do you know that they really aren't going that fast or rather having such a burst in speed that they're going about as fast and the misread in speed is from the way you're determining it? And what about little shortcuts and things that cut the distance between the waypoints/checkpoints/whatever you're basing it on? Things like google maps calculate the distance as if you're doing very rigid sharp turns and not curving over curners going ~50, you know?