My truck weighs about 3000 lbs. and gets about 20 MPG.
My motorcycle, which weighs about 500 lbs. should then get about 3000/500 *
20 = 120 MPG. It doesn't.
What is the flaw in my reasoning? Why isn't motorcycle gas mileage better?
Two possibilities that come to mind immediately are:
a)Car engines have historically been under more pressure to get more MPG, so there is more engineering maturity there.
b)MPG is related to difficulty in pushing the object through the air, not necessarily to weight. Motorcycles may not be especially aerodynamic compared to cars, so they may take comparatively a large amount of power to move through the air.
Yes and No; it's because motorcycle engines produce the most power for their weight at very high RPM (because they typically have only two cylinders), and the displacement of those cylinders is huge relative to a multi-cylinder car engine, in order to generate sufficient torque at low RPMs to get the bike + rider moving.
For example, a Harley might have 1,450 cc displacement that produces 80 hp. That's almost the same displacement of a 4-cylinder Honda Fit engine (1500 cc) that produces 117 hp.
So, for roughly the equivalent amount of fuel burned, the bike is 32% less efficient. However, the bike + rider "only" weighs about 800 lbs, whereas a Fit with the same driver is over 2,700 pounds, so 1/3 of the work needs to be done to get the rider/driver from place to place.
All of it, I'm afraid. The very premise that there should be a fixed ratio of weight/MPG is quite completely wrong.
MPG depends on a *lot* of things, among them mass of the vehicle, the wind drag of the vehicle, the type of fuel, the type and design of the engine, the way you drive it, and what the machine was primarily designed to do (cruise, carry or crazy?).
The mass of the vehicle only has an effect during acceleration, but none while braking or cruising at constant speed. So the less you brake and accelerate, i.e. the less aggressively you drive, and the more time you spend cruising on highways instead of stop-and-going through city streets, the less effect mass has on MPG.
Among other things, that's why hardly any vehicle owner achieves the published MPG figures in real life lest he tries quite hard to do exactly that, rather than just drive the thing. Contrary to many people's public opinion, those official MPG numbers are not figments of the marketing department's imagination. They're real measurements (required to be by law, in some countries). But because they have to be comparable, they are acquired in a very strictly specified standard cycle of operation, which is limited in terms of acceleration and top speed such that absolutely every vehicle (even a Prius, or a fully loaded Beetle) can do it, and with *every* non-necessary power consumer (AC, lights, radio --- everything) turned off.
Two reasons at least:
1) you may be looking at the wrong motorcycle :-)
2) nobody really cares about the fuel efficiency of dedicated fun vehicles, most definitely including motorbikes.
I thought we were talking about motorcycles not two wheeled agricultural vehicles.
Or for example the new Aprilia RSV4 has 999cc displacement that produces
180hp.
Motorcycles generally have poor mileage because they are not very aerodynamic, they have relatively huge engines tuned for performance not economy (compare mileage on a car with similar power to weight ratio, can you find one with 1bhp/kg?). And lastly no one really cares.
The tractors I've been around recently all ran much more quietly and smoothly than most of HDs I've seen.
Almost all half-way decent motorcycle engines produce at least
100hp/liter. The fact that an HD 1450cc engine only produces
80hp is a testament to the fact the HD's are all about noise, chrome, and vibration. (And proving how much money the owner has to spend).
The ones who do care tend to ride smaller bikes/scooters that do get pretty good mileage.
Sorry guys. I made a mistake with my newsreader and meant to post this to rec.motorcycles. I did make a duplicate post there.
I didn't make this followup post immediately (to admit my oops) because I figured engineers might have some insight, even if they aren't motorcycle fanatics.
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