Is the delphi wireless? Non-digital wireless can be subject to interference (producing wacky speeds). Otherwise, cyclce computers can miss detecting wheel rotations (producing low speeds). A low battery in a normal computer would generally report too low speeds.

GPS computers determine speed from position (none of them use doppler). GPS is more accurate for higher speeds. The Garmins have an option to measure speed from wheel rotation, which works when therewith is no GPS reception and it's more accurate too.

Regardless, I don't get odd speeds from my 800, used without a speed sensor.