Section 107.51(a) says the groundspeed of a small unmanned aircraft may not exceed 87 knots (100 miles per hour). That parenthesis is the whole conversion. Eighty-seven knots and 100 mph are one limit written in two units. The rule measures speed over the ground. An indicated airspeed that is still under 87 knots does not legalize a groundspeed that is over it.
The number, and who it binds
The opening sentence of §107.51 binds both the remote pilot in command and the person manipulating the flight controls. A supervised trainee on the sticks does not create a faster legal aircraft. The remote pilot still has to keep the flight inside the limit.
Read the distractors as unit errors. Eighty-seven mph paired with 76 knots swaps the unit and invents a companion figure. One hundred knots treats the mile-per-hour number as if it were already knots. One hundred fifteen knots is another invented conversion. The regulation states 87 knots (100 miles per hour). If a stem says the Part 107 groundspeed limit of 100 mph equals how many knots, the answer is 87 knots.
The limit is not confined to controlled airspace. It is not a second cap that also requires airspeed to stay under 87 knots. There is one speed figure in paragraph (a), and the word in front of it is groundspeed.
A tailwind that pushes the aircraft through the limit
A remote pilot flies downwind with a 20-knot tailwind. Indicated airspeed is 75 knots. Speed over the ground is 95 knots. With respect to §107.51, the operation is illegal because the 87-knot limit applies to groundspeed, and 95 knots exceeds it. The legal reading is not "airspeed and groundspeed must each stay below 87 knots," because the section does not write an airspeed cap. The legal reading is also not "75 knots of airspeed is under the limit, so the flight is fine."
Turn the same aircraft into a headwind and the arithmetic can go the other way. Eighty knots indicated into a 15-knot headwind can be about 65 knots over the ground, which is inside §107.51(a). The identical pitch attitude downwind can put the groundspeed through 87 knots. Watch the groundspeed, not only the number that looks like airspeed. A moving control station does not suspend the limit either. A flight that is otherwise allowed from a moving vehicle under §107.25 still fails if the aircraft paces that vehicle at 92 knots. That interaction is on the moving vehicle page.
The rest of §107.51 is not the speed rule
Paragraph (a) is speed. The same section also limits altitude, visibility, and cloud clearance, and the test will offer those numbers as if they were speeds. Altitude is 400 feet above ground level, unless the aircraft is flown within a 400-foot radius of a structure and not higher than 400 feet above that structure's immediate uppermost limit. How that structure exception is measured is on the 400-foot rule page. Flight visibility, observed from the control station, must be at least 3 statute miles. The aircraft must remain at least 500 feet below the clouds and 2,000 feet horizontally from them. Those cloud and visibility figures are on the visibility and cloud clearance page. None of them is a groundspeed.
Section 107.51 is on the waiver list in §107.205, so a faster groundspeed is something the FAA can waive when the safety case supports it. It is not a limit the pilot waives by staying in Class G, by flying downwind, or by launching from a moving truck. The list of waivable rules is on the rules subject to waiver page.
The trap
Pilots remember "87" and then attach the wrong unit, or they remember "100" and call it knots. The repair is to say the phrase the way the section says it: 87 knots (100 mph), groundspeed. The scenario trap is subtler. A tailwind makes the comfortable airspeed the illegal groundspeed. If the stem gives you both numbers, use the one over the ground. Ninety-five knots over the ground is over 87 knots, even when the airspeed readout says 75.
Say the limit once before you fly, in both units, and put the groundspeed where you can see it. Then the conversion item and the tailwind item are the same fact.
Drill the speed items in the free practice test. The §107.51 lessons in the course are at pricing.