Just from informal tests, it seems the atmega pins will signal a 1 on a rising signal that reaches about 2.5 volts and return to a 0 on a falling signal of about 2.2 volts.
You realize that we don't actually have a working diesel version as of this writing? I have been casting about for an old school mechanical diesel victim though.
The information we need to get from the car:
There may be another speed signal, or you can use a transistor to amplify what you have there to TTL levels.
Throttle position, might repurpose a TPS from the junkyard or parts store.
rpm, maybe an infrared transmitter/receiver positioned so they can "see" the holes in the pump pulley.
Plus lots of patience while we figure it all out