They are slow to catch up, some more so than others. If you go by the top speed recall, that can be decieving. I've made long runs at 92 mph with the unit bumping 93 mph occassionally, but once during the run somewhere over a 10 mile stretch, the unit will get an erroneous reading of 110 mph for a split second. If you go by the top speed recall, you were doing 110. I think GPS is accurate if you stretch it out and just keep an eye on the instaneous reading (or have a passenger do it).