So you still need an accurate modern vehicle to compare against. That is why I suggested following a buddy doing 60mph and compare your speed to his. I have an enginerd cousin who works on radar. He says even though the GPS is way superior to what it used to be, you will still be off by 1 or 2mph. And you MUST maintain an even speed or it will be wrong. He is also old school and suggested using highway mile markers and a stop watch then do the math. [that is what we eventually used on the boat to determine the speed. We got 55 mph over a measured mile on the water] Another cousin in PA says the State Police will give you a speedometer test on rollers for a small fee. Then you will be as accurate as a State Trooper car. His bone stock Ford Fusion car was off by 4.5 mph at 70mph. That's a speed camera ticket in the mail. This same cousin has a 1969 Barracuda and they don't make gears or an adapter that can correct his 5:38 rear gears. He then did the math and put some white stripes on his tach to know when he was at 30 and 60 mph. Finally, I don't know if you guys have these in your states, but we have these courtesy radar set-ups usually near schools and stuff. They say "Speed limit XX- your speed is__" I use those whenever I can because they are also State certified to be accurate. I had a cop stop me to ask why I was driving past the school so many times. When I told him I was calibrating my speedometer he was so impressed he didn't give me a ticket for doing twice the posted limit.