There's no law stopping people from doing this. There are enough startups making vehicles, enough people swapping designs on the internet, etc. Makerspaces are springing up all over the place, people are learning how to weld and fashion things from metal using lathes and suchlike, and 3D printers are becoming ever cheaper and more capable.

Yet it hasn't happened.

Closest I've seen is a rather unsafe and questionably legal add-on fairing (and frame/gearing modification) for a 125cc Honda underbone type motorcycle that increases its top speed from about 55-60mph to 85-90mph, and its potential economy from about 90 to 180mpg (US), though not both at the same time.

But that's a single (twin, at a very extreme push) rider, single-track experimental vehicle which would, likely as not, end up killing you.

Methinks the statement is somewhat rash. There are limits to what's achievable. Eventually you'll hit an entropic wall that governs how far you can move a certain mass, especially on a planet with a relatively thick atmosphere and moderately high gravity (thus beset with both aerodynamic and frictional/rolling resistance), at a reasonable speed (say, at least twice as fast as can be comfortably maintained on foot or, better yet, by pedal-power), with a given amount of energy. Electric vehicles face this problem just as much as hydrocarbon powered ones, and it's the biggest contributor to range anxiety after the currently terrible state of battery technology.

Then again, Volkswagen - one of the biggest and generally most proprietary-minded automobile manufacturers in the world - claim to have come up with a practical* 200+ MPG car in the form of the XL1. So it might be achievable, even if ironically enough by the travails of big business rather than countercultural open-sourcerers. If only they'll actually, finally, bring the damn thing to market so the claims can be more widely verified...

* for a certain value of "practical" - think a modern take on the bubble car, with a similar passenger and luggage capacity, acceleration/top speed performance and visual profile versus other vehicles, but somewhat better comfort, handling/braking, safety and overall fuel economy... and an unreasonably high price considering what you actually get.

It's faster and more comfortable than using a bicycle or a small-engined scooter, but only just. Most of the improvement is due to it having a roof, radio and heater, and being less likely to tip over on a slippery curve...

But, if you're after ultimate economy, this is the sort of sacrifice you have to make. All the whizz-bang technology in the world crumbles in the face of Colin Chapman's classic "simplify, then add lightness" philosophy. What works for making sportscars faster (without altering the engine) also works for making regular cars more efficient. There may be no replacement for displacement, but there's also no replacement for a low kerb weight and a minimal frontal cross-section and coefficient of drag, however fancy your 6-stroke lean-burn Atkinson cycle low-pressure supercharged diesel plug-in hybrid system may be. Modern cars are loaded up with heavy passenger safety and comfort equipment. You want to get around faster than can be achieved by your own muscle power alone, whilst burning the minimum possible amount of fuel? Either get on the bus, or buy a 50cc monkey bike...

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