the Kawasaki KLR 650 gets a hair over 50mpg for me, stone simple to work on, and carries me with cargo no problem. (I am 260lbs)

However, the bike is a bit tall, even with lowering links and such, so it might not be an option.

I like bikes in the 50+mpg range.

For an inexpensive cruiser that is peppy, comfortable, fairly light and gets 48 MPG the Suzuki Boulevard S50 (also known as the Intruder VS800) is a good choice. I have no problem commuting on my '94 VS800 or riding with friends for a couple hundred miles. The chopper style has a seat height of about 27.5 inches. 480 lbs and a low center of gravity. Uses 87 octane fuel and has shaft drive. I have been very happy with mine. Out of production since 2009 you can find them with low miles on CL. The throttle is responsive but not scary, but it is a powerful engine. Mid to low 13s in the quarter according to magazine tests. I just know I can spank the big Harleys I ride with, although it tops out around 90 because of it lower gearing. Anyways, I don't mess around much, I just enjoy riding in the mountains and highways. Never has a problem with altitude. Rep as very reliable, although they all develop a distinctive click due to forward cam chain stretch.

http://www.motorcyclecruiser.com/roadtests/budget_v_twin_bike_comparison/

http://www.topspeed.com/motorcycles/motorcycle-reviews/suzuki/2009-suzuki-boulevard-s50-ar43790.html

You might like some of the new super scooters, this Suzuki Burgman would make a great commuter bike, has lots of room pockets and storage space, economical and comes in 200/400/and 650cc versions.
http://www.topspeed.com/motorcycles/motorcycle-reviews/suzuki/2014-suzuki-burgman-200-ar160829.html

I traded off my 2008 Honda VTX 1300C for my Honda Goldwing F6B. Both got around 40 mpg.
Just talk to folks on forums and get their opinions.
Metric cruisers from 600cc up will get up to 50 mpg I'm told. Also, a shaft drive takes a lot less mess and maintenance.
I know what mpg my bike will get. Learning to properly shift helps in keeping the gas mileage up also. You run the gears up high on rpm's and you will burn gas.

Look for one '05 or later. The seat height is really low, you could put your feet flatly on the ground. They run very lean from the factory, on Friday I drilled the main jet on one 6% larger, sorry I can remember what the number of the drill bit was. Most shops just replace the main jet with a larger one, but I use a propane/natural gas flow chart to determine how much fuel is being fed.

By looking at the plug color I know how much larger or smaller a jet needs to be.

The 750 Shadow properly jetted pulls very well in low r's and doesn't fade out when you come up on the power. It easily runs Interstate speeds, yet because it is low and light it handles very nicely. On the street where our shop is I was doing figure 8's with it - the street is a residential street, two lane with no shoulders, just to give you an idea of its flexibility.

We work on all the metric bikes, sport bikes, cruisers, scooters, you name it. I really like the Shadow, but everyone is different. There really isn't a bad bike, something might be a bad bike for you but it would be perfect for someone else.

Good luck in your hunt!

Mine gets about 44mpg. It's a low-slung frame, Harley knock-off, so it might not be too tall. Not sure. You probably don't want one that old, but the newer models almost certainly get better mileage.

Fits her perfectly. Single cylinder, belt drive, light weight, easy handling. She rode it from Seattle to Malibu and back a couple of years ago, a 3,000-mile round trip, and was pretty comfortable. Our longest day on that trip was a little over 500 miles. I think she gets around 50mpg on it, on the road, less in town.

This was her second bike; she learned on a Suzuki 250, rode that for a couple of years, then moved up a bit. She's been extremely happy with the 650 for 7 or 8 years now. She's a daily commute rider; as long as there isn't ice or snow on the ground, she rides.