The cutoff gets it's information from the vehicle speed sensor (VSS) which outputs 8000 pulses per mile.
The speedometer in older 95 Contours uses a cable which I'm pretty sure is 1000 revolutions per mile (driven by the same gear as the VSS). Newer 95 Contours and all subsequent years use a speedometer that gets it's information from the VSS, same as the cutoff.
Despite this, the speedometer can be inaccurate. If you really want to go through the trouble, hook a digital speedometer or a frequency counter (some multimeters have built-in frequency counters--you'll need to do some math to convert the reading in Hz to miles per hour) to the VSS--I'd bet you'll find that your analog speedometer is inaccurate.
