Electric Bikes - Need higher gearing

I have an older, like new, low end mountain bike that I installed an electric hub onto to use for commuting. The problem is the bike is geared too low for street use especially with the electric motor assisting. I currently have Shimano SIS gears. The crank has 28, 38, and 48 teeth. The rear is an MF-HG37 FREEWHEEL with 14/28 gearing. What is my cheapest option to improve my gear ratios for increased speed? I really only need the high speed in one gear, for the other gears, I will take what I get. The best solution I can find is to install a cogset with 11 teeth on the smallest cog, but they are apparently pretty rare nowadays. I am open to any and all options as long as the cost is low. My only goal is to increase my top cruising (pedaling) speed from around 18-19 mph to around 22-23 mph or more.

Note: My motor will propel the bike up to around 20 mph on level terrain, no wind. I would still like to get some exercise out of my commute which is why I am looking for a higher gearing option.

First you need to find your cadance rate. That varies with age, weight, height, etc. Then you can use a the calculator at Harris web site to find the appropriate gearing for bike.

http://sheldonbrown.com/gears/

For rear hub motors, you are limited to freewheels. Best to look for vintage parts on eBay. There are some 11-34 Shimano MegaRange selling on eBay lately. Problem with this freewheel is that the gearing is too wide at the 11 sprocket side, so its not ideal for the same cadance, as the road speeds are too far apart.

Going from a 14t to an 11t is a HUGE difference.

I suppose that as long as your motor has enough power, you will be able to go much faster than 23 mph IF you can find an 11t cog.

Does your motor push you to 23 mph already? ??

if your motor does not drive the gears, then the only diff will be your cadence...

The best solution I can find is to install a cogset with 11 teeth on the smallest cog, but they are apparently pretty rare nowadays.

Buy a freewheel with an 11 or a 12 tooth cog.

12tooth is a 16.7% increase in speed for you. 11tooth is a 27% increase in speed.
(assuming your pedal cadence stays the same)

Thanks for all of your responses. Yes I agree the best solution is to by a freewheel with 11 or 12 teeth. Problem is they don't make them anymore and extemely hard to find.

This is all water over the bridge now as I slightly damaged my fork. The bolts loosened up on me and the motors axel opened up the right hand dropout about 1/4". Not enough to make it unusable, but bad enough so I don't trust it for motorized use anymore. I briefly pondered replacing the fork, but with the gearing issues I am experiencing I decided I am going to go ahead and buy a new (or used) bike.