If you are concerned with navigation get a WAAS capable unit. If not, they all do pretty well at measuring top speed. And most have a top speed saving feature. I personally am a Garmin fan, I own two (hand held and dash mounted chart plotter). The both show speed with immediate changes noted and I knw they measure WELL over 80 mph. I've used my hand held to get top speeds on land vehicles at times too. To date Rambunctious has had mine the fastest while in Europe for work. Something like 123 mph or something.