[Ray 911s]

You didn't mention what year - is your speedo mechanical or electric?

I only have half of an answer, since I know nothing of the electric jobbies. On the mechanical type that have a flex cable running from the gearbox to the gage - absolutely your speed and distance measurements are directly linked.

There used to be a device called a "half-miler" which was a gear reduction coupler that could be installed between the gearbox and flex shaft. It reduced the odometer readings by 50% - but you had to remember that your speedo was also reduced by 50% so a 25mph reading meant you were travelling at 50mph.

Again, I know nothing of the newer devices that do not utilize a flex shaft so I would not infer anything about the link between mph and miles on those devices.