Re: 18 To 20 Tooth?
i usually run a 36 in the front and an 18 or a 19 in the rear.
in the front, a smaller gear makes it easier to pedal, and in the rear, it's opposite, a larger gear makes it easier.
another plus about having a smaller gear in front is it gives you more room for the motor. and with the added weight, when you do have to pedal, it's not a struggle.
i have an assortment of rear cogs laying around, and since i don't run a tensioner, i find that going from an even number to an odd number of teeth will take out slack from the motor chain by moving the wheel forward or backward slightly. you can do the same with the front sprocket, too.
as far as the braking, scotto's post is more or less correct, but it's affected more by lowering the front sprocket gearing. going from a 44 to a 36 will give you a lot more leverage than going up 2 teeth in the rear. it sounds backwards, but with a smaller front sprocket, you don't have to pedal back as much, so the brake will grab faster.
changing out the rear only by 2 teeth won't really have much of an effect. i mean, there is, but if your brake works now, i wouldn't worry about it.