As David Simmons has pointed out, the difference in speed based solely on altitude is insignificant.
But “airspeed” vs “groundspeed” is a completely different animal, as iwakura43 mentioned.
I think that the OP’s question has been answered by the altitude mathematics, but allow me to clarify this part:
Note: Simplified discussion ahead!
This is most likely groundspeed. It is the airspeed of the aircraft corrected for many factors, including altitude and wind. Without getting too technical, air density decreases as you climb in altitude. Airplanes measure airspeed with instruments that measure the flow of air into them (called pitot tubes). As air density decreases, so does the Indicated Airspeed. If an airplane climbs using a constant speed climb (meaning constant Indicated Airspeed) it must actually fly faster in order to maintain the same Indicated Airspeed for increasing altitude. The airplane’s actual airspeed corrected for altitude is called “True Airspeed”.
There are many ways to calculate True Airpseed (TAS), including straight math, “whiz wheels”, and aircraft components. Most modern jets have TAS displays. As an example a jet cruising at 250 knots Indicated Airspeed at 35,000 feet might have a True Airspeed of 410 knots. This means that in a no-wind situation the aircraft would be moving at 410 knots across the ground.
One knot = one nautical mile per hour. Aviation distances are in nautical miles (except for visibility, for all of you Doper pilots checking my facts!). One nautical mile = 6000 feet. One statute mile (what everyone else is used to) is 5280 feet. This is where the difference between knots and MPH comes into play.
500 Knots = 500 NM/Hour= 500*6000ft/hr = 3 million ft/hr
3 million/5280 = 568.18 Miles Per Hour.
So 500 knots = 568 MPH in a no-wind situation. Add in a headwind or tailwind and the speed will increase or decrease, and will become your groundspeed.
Since wind is in knots, the correction is made before the conversion to MPH. For example, 500 KTAS (Knots True Air Speed) and a 50 knot tailwind equals 550 Knots Groundspeed. This equates to 625 MPH Groundspeed.
And for the record, MPH is used only when relating to non-flying folks like passengers or news audiences. Knots are used exclusively between pilots and controllers.