I was out today and took my Garmin Montana.  It's a hand-held device which has multiple uses.  I mainly use it when cycling. https://buy.garmin.com/en-GB/GB/p/523640 I did three trials at an indicated 40mph, 50mph and 70mph.  I tried to do a 30mph but it wasn't very easy due to traffic etc.  I didn't want to do 30mph on the open road because I would have held the traffic up. I did a total of 52.92 miles as recorded on my Montana, and the car said 52.9 miles.  Spot on, I reckon.  H