What model is your car?
If it is a PH1 it still use a pickup on the gearbox if it is PH2 or newer it use a signal generated by the ABS for the Speedo.
In this case, neither, I used this method on my 116 Alfetta.
I'm assuming that if the Sender is not
using the ABS as the Pulse Generator it will be very similar on the 916 cars.
Just in case you think I'm losing my marbles ( I may be but not in this particular instance
) I'll explain the theory.
The Sender Unit is an electronic Pulse Generator, a rotating metal spoked wheel driven by the Gearbox affects the magnetic field of a coil, causing pulses which are then smoothed and amplified by the Electronic circuits within the Sender.
Using the File creates a very, very crude Pulse Gen which is enough to get the Speedo Needle to move about, in the case of my '6 a rapid scraping produced a jerky movement with a max reading of about 40 MPH on the Speedo.
Whether or not it will work on any of the 916 cars I don't know, but I think it should as long as the ABS is not involved. It was an old school auto 'leccy that showed me this little trick, he still uses it ! Saved me a lot of grief in diagnosing the problem but I was then left with the problem of fixing it..........