Motors have a fundamental characteristic called the Kv, which is how many volts per RPM it generates. The voltage it generates cancels out the battery voltage, so if you had a motor with a Kv of 0.184 it would generate 24v at 130 RPM, which is approximately 10 mph with a 26" wheel. If you have a 48v battery, at 260 rpm there is now 48v -24v=24v going into the motor to make power. When the motor spins at 260 rpm (20 mph), it's generating 48v, so you have 48v - 48v = 0v to power the motor. In other words, the motor will not be able to spin faster than 260 rpm with that battery, but if you gave it 72v, it would spin up to a maximum of 30 mph.
Hopefully, you can now see that the maximum speed of any motor is dependent on it's Kv and the voltage in the battery. Don't forget that the voltage of a 48v battery is 54v when fully charged and 40v when empty. Obviously, according to the above explanation, the motor's maximum speed will change in proportion, so you'd get 25 mph when charged and 16mph when empty.
Those speeds are maximum theoretical no load speed. There's one thing more to consider. When the motor is spinning at 250 rpm (19 mph) with a 48v battery, it'll be generating 0.184 x 250 = 46v, so you have 48v-46v = 2v left to power the motor, which isn't enough to get any meaningful power. It's enough to spin the wheel in the air, but not enough to drive you at 19 mph, so you'd slow down until you got to around 16 mph, where the net voltage would be high enough (10v) to push the maximum current theough the motor and give you enough power to maintain that speed.
To summarise, every motor has it's own Kv, which is fixed in the design by the designer, who can make it whatever he/she wants. If you don't know what a motor'sKv is, you can't determine how fast the motor will go. Sometimes the sellers will say 48v 260 rpm, which is enough, though they never seem to understand the importance of this information and they'll say something like 260 - 320 rpm. It can't be both. It's one or the other, but what they might mean is 260 rpm at 36v and 320 rpm at 48v.