You can treat a 3700kv to be a 'typical' 10.5R motor. So gearing should be in the 5.2 to 6.5 range.
However, many factors will affect gearing: ESC, battery, car weight, size of course, outside temp. etc...
It is best to use a temperature gun and measure the motor's temp after a few laps. If temp between 140 to 160F (or 60 to 70C) then your gearing is good for that course.