to get accidentally inflated numbers is easy. the sampling rate and the way the device gets its signals from the satelites ends up making gaps in the data. the device averages these to get your speed. depending on the way the machine figures out and stores the max value you get easily get inflated numbers. think of the data as a zig zag that averages around your average speed. some of the data points will lie above the average. if the machine uses these to compute the max, it wont be right.
for a test we tried it on a bike that maybe does 40, max GPS speed was 83. what did i do wrong? i turned it on. reset everything. guy went riding. stopped. look at max speed. 83. now you are going to have people all over the country doing this and thinking they are the fastest. so i dont trust it.
side by side or measured distances with light sensors to determine time for me.