You have a 15A controller, good for 15x36 = 540 watts peak. When the battery is fully charged, it will be close to 40V,so that is 600 watts.
If your battery has the current, you could put on a 20A controller and get up to 800W. That will not really change top speed, but could help acceleration. There is usually limited room to put in a larger controller, and it is an unnerving experience if you are not an electrician or have wiring skills.
You can buy an inexpensive bike computer to give you speed and distance. Also get a good bell/jorn and some light. In my opinion, a better purchase. A 36V ebike will almost always be limited legally to 20 mpg enyway, and it's probably limited by the design to the same speed,
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I've put together many DIY hybmotor ebikes with various motors and batteries, and .they all seem to top out around 20 mph at 36V. Higher speed need more voltage.
