Jamis ebike internal hub sprocket question
 

I have a Jamis commuter with an internal hub 8 speed Shimano Alfine rear and a front hub motor. I just use it around my neighborhood and for short distance shopping. It has a 23mph top speed with full power but I can't pedal fast enough to add any muscle power when I'm flat out (the front crank just spins freely at top speed). I'm an old dirt bike rider and we frequently changed sprockets for more speed or more grunt. The bike currently has a 42 tooth front and a 20 tooth rear sprocket. Can I gain a little more top end (while pedaling) by changing to a 16 tooth or 18 tooth rear? Rear sprockets seem to be cheap and available on Amazon. Be gentle; I'm a noob!

Not an expert with IGH, but you should be able to go smaller in the rear (easiest since you won't need to add to the chain) or bigger in the front. Probably there is a lower limit in the rear.

I think 16T is the smallest rear sprocket I can fit. Should I go 16 or 18?

Find an online gear calculator like the Sheldon Brown one, and plug in your current variables. Then switch out the rear sprocket to 18 and 16 and see how those numbers compare. If you're currently not finding a need for the bottom few gears, I'd probably jump right up to 16. Although rear 3 spline sprockets are so cheap that you could get both 18 and 16. First try 18, and if not enough then go to 16. This way you can keep shortening the same chain.

Excellent advice IMO.

Thanks, I'll give both of them a try.

Order both sprockets
 

They’re so cheap that I would just order both.

I went with 16T. When my hub motor is running at top speed there is still resistance on the pedals so I can pedal hard and increase my speed up to 35 mph. If I thought a 14T sprocket would fit, I'd try that.