OK, so I've got my MPGuino working! It's a freeduino-based version.
After many attempts I think I now have the right connections for speed and injector.
I set the speed pulses to 16000 to get a correct mph. For the injector calibration I set it to a very large number, for now.
At a steady speed it displays a believable mpg. However if I accelerate mpg goes very high, and if I idle mpg drops. I reckon this is because the injection pressure is varying so much. So around town it's of limited use as it stands, but most of my miles are on open roads so we shall see!
This is the signal that the mpguino sees for the injector. Sign is reversed; positive on the scope was connected to ground on the arduino. Is this going to be OK,given that it does not return to zero volts, and there are effectively 2 "leading edges" per injection?
2.0HDi injector pulse at about 1900rpm (after 100k resistor)