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2,496 Posts
Discussion Starter · #1 ·
I've plagiarized this from another (private) list, which may be of some interest.
(Yes, the author is an engineer. Automotive actually)

The relationship between fuel economy and speed for the ST1300 will be
similar to that for other motorcycles of similar power. As speed
increases, power demand increases exponentially because aerodynamic drag
is related to the square of velocity. At 70 mph, a typical sport
touring bike requires about 12.3 horsepower at the rear wheel, which
translates to about 14.5 hp at the crankshaft after accounting for
drivetrain and tire loss. At 90 mph, the rear wheel power increases to
about 25 hp and the crankshaft power is about 29.4 hp.

Even at 90 mph, the crankshaft power required in only about 20% of rated
engine power, so the engine is still heavily "throttled" and the
throttling losses reduce its efficiency to about 0.60 lbs of
fuel/brake-horsepower-hour. At 70 mph, the engine has even higher
throttling losses and its efficiency is reduced to about 0.64

Doing the math, the fuel economy at 70 mph is about 45 mpg and the fuel
economy at 90 is about 31 mpg. That's representative of steady speed
cruising over level terrain at sea level with no wind. On a bike with
an exhaust gas oxygen sensor, fuel economy increases at higher altitudes
and over slightly rolling terrain. Fuel economy obviously decreases
with net elevation increases or headwinds.

ninja machinist
3,103 Posts
Cool! What I'd like to know is how much of an effect the secondary injectors on my bike have on fuel economy. When I was in alberta I made the trip from Calgary to Lethbridge in exactly two hours at an average speed of about 145-150 km/h. And I got the same fuel mileage that I usually get from stop and go riding. I think I just barely squeezed out 235kms from my tank.
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