3100 sounds pretty right to me for my old xr4i, diesel P100, would most likely be on the 3.14:1 diff, so would be to establish engine rpm at any road speed,
60 X rpm / wheel revs(mile) X diff ratio X gearbox ratio
You can also use the formula's to work out speedo errors as well , ford traditionally use either 6 or 7 teeth drive gears.
I have no idea of the box ratio for the diesel otherwise I would have shown this.
tabetha