One other thing to take into consideration is prop slip. If you use the equation to figure out your max speed. RPM X Prop Pitch divided by gear ratio X 1056 = theoretical speed. Most cases you will get around 10 percent prop slip.
4500 X 19/(1.86 X 1056) = 43.53. That would put you between 39 to 43
Also you will turn fewer RPMs in the summer months because of the humidity and heat make it harder for the motor to breath.