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I'm currently driving a Honda Hornet 250 CC (because it's the engine size limit on a provisional license in New Zealand). Once I get my full license I want to get something bigger, possibly a Honda Hornet 900 CC.

Does increasing the engine size by more than three times mean I use three times as much fuel? I'm pretty sure this isn't the case but I don't know what the real answer is or why.

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It depends on many factors, such as modernity of the engine, whether it is fuel-injected or carburetted, gearing set-up, driving style, preferred mode of operation (city, or motorway/rural).

In a similar conditions my FI 250cc Suzuki TU gets twice better economy than my father’s carburetted 800cc Kawasaki Vulcan (2.8 L/100 km v. 5.4 L/100 km). Weight is probably the biggest factor. In my example the displacement goes up more than threefold, but the weight of motorcycle with rider only goes from 235 kg (150 kg bike + 85 kg rider and gear) to 340 kg (245 + 95) — only 40% increase.

In the city I much rather prefer my nible standard, but for the sustained motorway journeys bigger bike would do a lot better (and shall get better economy in comparison to city riding, as my small bike’s RPM get too high going close to 90–100 km/h and up).

To check specific models’ real-world fuel economy, browse all motorcycles at Fuelly.

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By physics yes, the cylinder capacity get increased by 3 times, so more fuel is required on each revolution/cycle (each time the piston goes down and fill the cavity of the cylinder with the mix of petrol and air).

By mechanics, not necessarily, both motorbikes must have the exact same configuration in order to the physics to be applicable. Both motorbikes must have the same gearbox and ratio, wheel size and cylinders and weight.

Usually 125cc and 250cc are motorbikes designed with economy in mind while bigger motorbikes are designed with performance in mind.

In my own experience a 900cc will be more expensive to run in the city or at low speed, but for some long journeys it will be close to the cost of running a 250cc

The difference is not that big at the end of the day if you give the motorbike the right use.

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P. S. As a stickler point, while your point is clear, terminology is suspect. Mechanics is a branch of physics so it always applies. Better to say it is not simple arithmetic of engine displacement numbers but comparison of multiple factors between two systems. –  theUg May 18 '12 at 16:03
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