Here is how I interpret Denny's words:
"check how much with a GPS and in my case it [the GPS ?] comes out to be right around 5 mph slower than he [the] actual speed [speedometer readout]."
For example: the GPS reads 50mph and the speedometer (Denny's "actual speed") reads 55mph.
Excuse me here: the GPS is really the actual speed.
The GPS is the real/actual speed and the speedometer is the actual read out, which has to be re-calibrated.
If I am correct (and I probably am not), take off the needle and put it back on, pointing to -5mph.
