I decided to get a Blitz dual turbo timer.

I haven't fitted it yet since I (wisely) decided to investigate how to do so first!

I didn't bother learning japanese just to read the book, so instead I spent a bit of time searching the web and managed to find enough information about the ECU in order to see which wire I needed to splice into.

The wire is the SPEED signal, which is NOT engine speed i.e. RPM. Does this mean the actual road speed?

This surprises me, since I thought that if the turbo timer is supposed to work out how much time is needed for the idle based on how 'hard' I've been driving then surely it should be monitoring the RPM?