Hey Bill, Nice build, thanks for sharing!

Your bike’s speed is going to be determined by your motor’s RPM. You can add as much power as you want, but the only way to change the RPM is to change the voltage or the motor’s windings. Sounds like your motor may be ~250 rpm at your voltage. A higher rpm motor would give you more speed (if you have the power) but also overheat quicker under heavy load (i.e. hills).

What the heck is a 6000ma 18650 cell? Like 2old said, the most powerful 18650 cells I know of are 2900mAh (Panasonic or Samsung). If you have a secret cell, please tell us!

48V54ah? That doesn’t sound right. 18650 Cells are typically 3.6v and 2mAh. This gives you 48 volts and 18Ah.

Voltage: Typically it might be 54 volts off the charger, and down to something like 36 - 42 volts on empty. What is your Low Voltage Cutoff (LVC)? Your BMS should cut the power around 36 volts. A 48v controller would typically have a LVC of something like 42 volts. Is your controller programmable?
Could be that one of those two pieces thinks your LVC is 49v.

Check that BMS & controller if you want to get the proper range out of that battery.