I have used Radar and Laser to check speed, they are both accurate and reliable they both have pros and cons, but in this application I feel they fall well short of ideal.as for vehicle speedos...they suck, right from the factory a 3-7mph varible is the norm, plus you have to figure in tire wear as well as proper inflation. this is why most officers wont write tickets unless you 10-15 over. (I know some write for 1 over) The biggest problem with GPS is the handheld units dont compensate well for the AOA that the jet is at, ie. the jet could cover 100m of ground distance but the net distance could be greater or less based on the plane it was travelling thus giving a indicated speed that could be higher or lower than true airspeed, plus the low cost GPS systems dont have the accuracy that 747Drvs aircraft do. B1 Driver pretty much nailed that, The most accurate timing systems for this application in my opinion would be point to point measuring systems, similar to law enforcment's VASCAR that can factor in all of theses variables. Just my two cents. also using overpasses to estimate your speed in a vehicle isnt all that accurate either.