Watch My Speed

Warns you with a tone when you increase your speed across each 5-MPH threshold.
If you are maintaining your speed well, you will hear nothing.
But when you leave the 5-MPH range you have been keeping within, you will hear a tone warning you of your increase in speed.
This simple approach needs no setup or interaction.
It will stay working even if you open another application on top of it.

The visual part is a digital speedometer that is useful as a check against your car's speedometer, which can be inaccurate.

There is also a visual analog speedometer with an added "speed limit" indicator that shows the top end of your current 5-MPH range.

As your speed drops below a range, a low tone will let you know, and the range indicator will also drop.