Should be Able to do 235 MPG
The Honda ANF125i Innova motorbike is a super-efficient bike to start with, but to bring it to the next level of fuel economy, from merely "super-efficient" to "holy crap efficient", it takes a certain level of ingenuity and craftsmanship, as well as a good understanding of the factors at play in how much fuel a vehicle uses. Allert Jacobs obviously has those qualities; he turned his 100+ MPG Honda (see photo of what it looks like unmodified below) into a 200+ MPG low-CD streamliner.
Honda ANF125i Innova. Not exactly same as bike below, but close.
The modified motorbike weights 319lbs (about 88lbs more than the standard bike). The engine is a fuel injected, four stroke, 125cc single, producing 9 hp (6.85 kW). A realistic figure of real-world MPG for the unmodified bike is around 114 MPG (though Allert got a 133 MPG average while babying it).
The modified bike, despite weighting 88lbs more than the unmodified version, gets over 200 MPG in real world driving (214 MPG during a trip in rather cold and windy weather, but more is expected in better conditions). This shows how important aerodynamics is for fuel economy. Sometimes it can be better to add a bit of weight if it's going to make the vehicle more slippery (and the benefits can be compounded by using taller gearing in the transmission; since you need less energy to push against air resistance, you can use gears that keep the engine at a lower RPM).
One of the things that reduces the frontal area is the lower seat and feet-forward seating position. This quote from Ecomodder highlights the importance of aerodynamics:
"Being a long time cyclist, motorcyclist and velomobile enthusiast, Allert intuitively understands what many people either don't know or greatly underestimate: the enormous impact of aerodynamics on fuel consumption.
Unlike the average person, he is not surprised by the fact that a typical car burns 50% of its fuel overcoming air resistance at just 40 mph (64 km/h). Or that the higher drag of a typical motorbike means half of its fuel is used to overcome air drag at just 15 - 20 mph (24 - 32 km/h)!"
More High-MPG DIY Vehicles
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