I was originally taught to do it with a sheet of paper and a pencil on the 50000 series maps we used for rallying 'Trap' the paper with the point of the pencil and 'roll' the paper along the route you wish to measure - Surprisingly accurate with a little time and patience.
Like Google Earth the only discrepancy that can creep in is with the width of the roads. From memory a 'Yellow' or a 'White' (both minor roads) scale up to something like 100 foot wide
I still use the paper edge and pencil method Bob, not having a wheel measure that suits the maps I use most. I try to follow road centres point to point, and as you say, the results can be very accurate with patience and a bit of care.
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