Flywheel modification

The electric conversion of the 1969 Saab Sonett retains the OEM transmission, but the new electric motor does not use a separate starter, so the ring gear around the circumference of the flywheel that originally meshed with the starter gear is now redundant. To save weight, the ring gear was removed.

  Drawing of the flywheel modification shows ring gear removal and drilled magnet holes

In a gas-powered car, the tachometer typically uses the electric impulses from the distributor to measure engine speed in rotations per minute (RPM).

The digital replacement tachometer in the Sonett EV can be “spoofed” to display RPM by placing two opposing magnets on the outer edge of the flywheel. These magnets are read by an inductive proximity sensor that produces a small voltage spike when its head passes within a few millimeters of the magnet.

With two magnets 180 degrees apart, the sensor will register two pulses per rotation. The digital tachometer can be programmed to properly display RPM based on this pulse pattern.