I am 6'2" weigh 280 pounds. I live in the mountains of Pennsylvania. Between these two situation I find my 80cc slowing down to 20 mph on the long hill climbs in this area. It was designed in the 1970s and looks like it, made in Mexico for the American Market. Web site:
http://powersports.honda.com/index.asp?bhcp=1Honda Scooters:
http://powersports.honda.com/scooters/Picture on my Moped:
If you notice I installed a windshield on the bike. I bought it from Clear-view windshield:
http://www.clearviewshields.com/They are a little slow at getting the windshield to you (They make them in batches so you may have to wait 2-3 months before they can ship one. When I ordered my they did NOT bill me for two months, and then only ship it to mw a month later, but I did get the windshield, just a warning to wait. They make windshields for a number of bikes, go to the website and see what they have.
As to my moped I have to make the following comments but I calculate I save 10 cents a mile when I ride it instead of my jeep. 10 cents does not sound like much, but given most people drive 12,000-15,000 miles per year that comes to $1200-$1500 per year (My Calculation assumed $3 a gallon Gasoline).
I avoid limited access highway, my moped is to slow (it goes about 35-40mph on the flats). I have taken it on what Pittsburghers called the Parkway East (I-376) between Downtown Pittsburgh and Monroe ville where it diverges into US 22 and the Pa Turnpike (I take US 22). US 22 is a four lane road but with US 22 is NOT limited access (it has a lot of red-lights). I am comfortable with the Moped on US 22, but not on limited access roads, mopeds are just to slow. Remember common roads are open to ALL traffic, which includes horse-drawn wagons, pedestrians, bicyclist in addition to mopeds. You have to take your stand by riding your Moped on theses roads.
Now, as to speed, the bigger the engine the lower the top speed, but the higher the MPG. I get about 90mpg on my 80cc moped (When I keep it below 25mph it went up to 110mpg). 50 cc will get about 120mpg, 250 cc gets about 60 mpg. My Brother has a 650 and get about 45mpg. I believe part of the problem is the weight of the rider (Which is a big factor when it comes to Moped as compared to Motorcycles and cars). Remember the more weight the bike has to carry the less fuel economy it will get (Women, who tend to be lighter then men will see greater fuel economy in a moped than a man). The larger the engine the less variation between riders (As the weight of the bike becomes the limiting factor NOT the weight of the rider). My Moped weighs 179 pounds, thus when I get on it I double its weight. This hurts its fuel efficiency but I still get 90mpg.
The downside is going up the local mountains. Most of the mountains climbs are slow gradual climbs , which my moped can handle, I have to dismount only on two hills, both residential and steep (The City if Pittsburgh, where the streets were, just laid down a traditional street pattern, ignoring the fact they were climbing the Monongahela escapement. Many roads in Pittsburgh were NEVER designed for automotive traffic (Horses have huge advantages when climbing steep hills, which is why most of the roads were designed for and then closed when horse traffic died out). I make this comment for except for these very steep exceptions, my Moped can take on almost any hill in this area. it may do so at 15-20mph but it will do that speed forever or until you get to the top of the Mountain.
I like my moped, people pass my all the time when I am on it. When people see you are on the road they will avoid you while the car drivers pass you but that, in itself not bad. Remember the studies on Red-Lights and Stop signs. Which is safer? If you ask people it is red-lights for they clearly see who has the right of way, but traffic engineers looking as the accident rates have long pointed out it is Stop signs that are safer. The reason for this is simple, people think stop signs on intersections are less safe, so they pay more attention and thus avoid more accidents. People with Red-lights rely on the lights and thus pay less attention to other traffic and get into more accidents. The same with riding your Moped, it may look more dangerous then riding a car, but once you remove people showing off with their motorcycle (A huge number) they are as safe as automobiles do to the fact people view them as dangerous and thus take extra care when passing them, thus Mopeds and adult bicycle riders are safer then Car drivers (Notice I use the term Adult Bicycle drivers, a lot of teenagers get in trouble on bicycles for the same reason Motorcyclist get into trouble, trying to show off, when used only as transportation the numbers of accidents drop).