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