A job change a few years back took me from a bikeable job location to one that is 30 miles away in a bike-hostile 'job center' area with no reasonably safe approach for bikes. Carpooling now, whenever there are only 2 people, I drive my 2 seater Honda Insight at 55mph on a three lane 65mph toll road where people normally drive 75mph. When the oil price shock occurred last year, I started noticing people pulling up behind me in a recognition of 'hypermiling' at a more reasonable speed. I would often end up at the end of the toll road with 20 cars behind me, much like a caravan. When prices began dropping, the line grew gradually smaller; now there are rarely times when cars line up behind me.

If I stick to no more than 55mph, I can achieve mpg readings in the high 70s to mid 80s. At 65-70mph, it drops to the low 60s to high 50s.

On the topic of motorcycles with full wrap-around fairings and smaller engines, see the link E. Swanson posted above to Craig Vetter's motorcycle mileage competition in the 80s. Note the care taken with streamlining on the results achieved.

550 lbs at 30 mph on a 6% grade would eat 2.64 hp from the vertical rise. That's neglecting drag and friction.

OLDFARMERMAC,
The Indian manufacturer BAJAJ makes a 125cc motorcycle that can attain up to
86 km on 1 litre of gasoline (202 miles per US gallon!)
The engine is a 4-stroke, 1-cylinder type with 2 spark plugs.

http://www.bdmautomotos.com/noticiasview.asp?key=22

A couple of years ago I dropped my highway speed on the commute from 80 (like everyone else) to the speed limit, 65. I noticed two things. First, was that driving was far less stressful, and second, was that my gas mileage went up by 10%.

"if somebody builds one of these superstreamlined bikes and powers it with a really good three or four horsepower diesel(I'm not such such an engine exists commercially yet)it could maintain a good speed , probably thirty five or forty up a long grade with a heavy rider."

OLDFARMERMAC,
The original VW 1-litre prototype is powered by a 8.5 HP naturally aspirated diesel engine.
It could attain 264 miles per US gallon (0.89 litre per 100km)
The weight of that vehicle is just 290 kg (639 lbs)

http://www.seriouswheels.com/cars/top-vw-1-liter-car.htm

I don't know about diesel engines, but good low horsepower gasoline engines do not exist commercially. My school's supermilage racer gets 700 mpg (basically a recumbent trike in a fairing), but the winner of the last Shell EcoMarathon (Americas) got 2400 mpg with essentially the same design, but with a better engine. Both engines were running on normal gasoline.

Assuming 200 kg mass, a CdA of .15 and a speed of 45 mph, and a grade of around 15 degrees to the horizontal, I got 2 kw as the required amount of power. If you assume 2.5 kW once rolling resistance and transmission losses are factored in, I estimate 170 mpg under those conditions. That's with 25% thermal efficiency in the engine, I have no idea if that is possible with a small diesel or not.

A little off topic but does anyone know how well current hybrids such as the Prius work in mountianous country?

I just zipped over the "hill" to San Francisco from Reno and back in my 2005 Prius.
(Hymotion pack thought it needed replacement, no more 75-99+ mpg on local drives and its switch LED was blinking at me).
While the engine strains going up out of Truckee to the pass, I can keep up with pretty much anyone who's not speeding too much.
Mileage drops of course, on the serious uphills at 65 going on 70 (by accident of course;-), I only get 15-20 mpg.
The nice part is once over the summit, it's 55+ mpg total from Reno to Sacramento (even with a dead Hymotion pack - which would have been used up halfway to Truckee anyway.)
Coming back uphill is only 45 or 50 mpg (Sacratomato elev. 25 feet, Donner Summit Eastbound 7227', Reno 4500 or so.)

Trips to Incline or Virginia City (winding roads) are speed limited by the curves, not the grades.

Not as peppy as the old Saab turbo to accelerate while at speed, but not an embarrassment.

Nice to have the engine brake at times, often going down from Donner Pass into Truckee with traffic I'll want to keep the speed down and the stock battery gets fully recharged fast, then I'd have to rely on the brakes really touching, so just shift into "B".

The Prius has been to Colorado (Vail) twice, once via (soon to be)Glacier(-less) National Park, several times to Oregon (Bend/Mt. Bachelor, then on to Portland), Flagstaff, San Diego via Palm Springs, etc. Jevons Paradox I guess, 71K miles on a 4 year old car, with a work commute of 7 miles one-way.

Only issue is in snow - clearance is very low, so I wouldn't try un-plowed snow deeper than about 5-6 inches. For this, I have a diesel Ford F-350 4WD, with about 2000 miles in 2 years, that has never been farther than the local ski areas or Lovelock, NV (90 miles East of Reno).

.. and I just voted for him again, having also seen him on the street as I walked my daughter to school earlier. Boy, it's nice being on foot.. I had two or three small exchanges with neighbors in our 3 block trek.

Time to get out the hammer and a blunt nail.

A couple decades ago I built a wooden tool box for a 1988 full size bed regular cab Chevy pickup. The design was accidental, in as much as materials at hand and tools needing shelter were the most important factors. That year GM put the shelf support in the bed so the tool box was elevated. I tilted its lid forward for drainage away from its rear facing tailgate styled door. The 4 foot front to back and just under 5 and half foot wide box roof tilted about 1 1/2 inches forward from bed level, extended forward to about an inch from the cab rear window the high rear edge of it was about the height of the center of the same window. I drove that truck for 188,000 miles and ran the same trips repeatedly both with the box on and off, depending on what load would have to be hauled one way or the other. For years I kept an accurate mileage log. It ended up that on the same 200 plus mile trip I consistently got 21 to 22 mpg with the box on and never exceeded 20 mpg without the box in either tailgate on or off configurations (though the latter was rare--I had a truck because I hauled stuff and the tailgate up or down generally facilitated that quite a bit). This was highway speed mileage and 68-72 mph with the box was often the best. I always thought it would be interesting to see just what location of a plane like my box roof would give the best performance at given speeds. That old chev was 4.3 litre vortec V6 with a 5 speed w/OD standard transmission and relatively mild standard 1/2 ton pickup rear end (the automatic trany rear end was steeper).