On a car (England) odo trip, what are the numbers measured in?
For example, if the trip says 215.7 the 215 is in miles right, but how is the .7 measured?
Car odometers work by counting the number of revolutions of the car wheels. In the old days, a flexible shaft (the "speedometer cable") was rotated at one end by a gear driven by an axle or a wheel, and the other end went in the speedometer and drove a bunch of gears . Some of the mechanism used the speed of rotation of the speedo cable to move a pointer showing the speed the car was going at. Another part drove a set of gears with numbers 0 to 9 printed around the circumference, behind a set of little square holes so that one number showed through each hole. These gears were designed so that each time a gear went round from 9 to 0 it drove the gear to its left around one tenth of a revolution. Typically they had had 6 gears (or cogs if you like) and a new speedo arrived at the car factory showing 000000.
The mechanism prior to the display cogs was designed so that every time the car moved one tenth of a mile (176 yards) the right most distance showing cog moved round by one number.
A new speedo would be fitted in a car on the production line and pretty soon the car might be moved a bit and after it had moved 176 yards it would show 000001 and after it had moved 352 yards it would show 000002 and so on. It was convenient to have the rightmost cog measuring tenths of a mile, even though miles are not a metric measurement. When the car had moved a whole mile the tenths cog would pass from 9 to 0 and the miles cog would move from 0 to 1.
A mechanical odometer with 6 digits showing tens of thousands, thousands, hundreds, tens of miles, miles, and tenths of miles would go right around from 999999 to 000000 when the car had done 100,000 miles. Not a lot of cars did in those days, so that was OK.
In my youth such a car was said to have "gone around the clock" and was usually not worth much.
Dishonest used car dealers would take a speedo out and use an electric drill inserted where the speedo cable goes in to drive the odometer back to show a lower figure.
Nowadays the speedos are electronic but it is still convenient to show tenths of a mile.