Torque can be thought of as a rotational force, which can achieved by a 'linear' force applied perpendicular or normal to a moment arm - hence ft-lbf, or N-m. In the motor, the piston (through a rod) imposes a force on the crank shaft, and the crank shaft produces the torque.
I am not sure of your question about a motor enabling 15 mph vs 20 mph. Do you plan to replace the motor or add a second motor?
To go faster one needs more energy/power as the resistance (mostly wind resistance) increases with speed - so more energy/power is required.
Also, the motor needs to turn fast enough or the gearing must be arranged to permit the driving wheels to turn at the appropriate vehicular speed.
You would also want the RPM the torque is being measured at so you can figure power. And since you would be well suited with more information in this case, a chart/graph would be even better.
So one way to interpret it would be to have a graph of torque vs RPM, from that you can create a power vs RPM (and vice versa). Now if you know the RPM of the motor at the top speed you can make a determination if the engine is simply at its top RPM (at which point the power is a non-issue) or if the resistance is high enough to prevent it from reaching maximum RPM the amount of power needed to maintain that top speed.
If the engine is making more power at a different point on the graph, you may be able to shift the gearing up/down to allow the power curve to better suit your top speed goals. Or approximate the power needed to increase the speed - I thought I had a rudimentary understanding of aero drag but some posts lately make me question that so I'll leave that for someone else.
Electric or gasoline powered scooter by the way? And the additional or replacement motor is what?
it is electric and it is additonal to the other two stock electric motors. There is no way I can gear down or up the two stock motors fo sure so I guess to achieve high max speed is by adding two more motors with smaller gear ratio.