Going along with sreid's prediction, it's important to
realize that current gasoline fueled engines have higher
potential that what we see in production. Here's a quote
from David Vizard on the subject, when reading remember
this was 30 years ago:
"In 1975 I was the lead engineer on an economy project. The
subject was a 1275 Mini GT. That’s a direct relative of the
original 1275 Mini Cooper."
"The goal was to see if we could get this car to do 50 mpg at
50 mph. Once that had been achieved it would be taken on a
3 month round USA drive at normal highway speeds (not fuel
economy test speeds which would take forever to go anywhere)
and see if we could get 50 mpg.. An independent test at
MIRA (Motor Industry Research Association) on a truly lousy
wet and windy winter day recorded 55 mpg at 50 mph and 99
mpg at 30 mph. OK that 99 mpg is real close to 100 but do we
all want to drive at no more than 30mph – no! Not only that
but that 99 mpg was achieved with a 1600 lb car. The same
size car today weighs in at over 2100 lbs and the average
mid sized sedan at 2800 lbs plus."