What aspects of modern motoring would you be prepared to give up to achieve 125 mpg fuel efficiency?
Carmakers have made huge strides in recent years to improve fuel efficiency, but they've had to balance these economy needs with the demand for ever increasing equipment and safety levels, and higher performance too.
So what would it take for cars to reach a real-world 125 mpg on the highway?
Air resistance is one of the biggest factors in highway economy, and surprisingly it's one area where we might not see a great deal of change.
A decade ago, it took compromised shapes like the original Honda Insight to achieve low-drag figures, but today's aerodynamic shapes are surprisingly normal. The Toyota Prius is most distinctive, but cars like the 2013 Hyundai Sonata Hybrid return a similar 0.25 coefficient of drag figure.
The biggest aerodynamic change could be reserved for small cars, which have a limited surface area over which it's difficult to tailor a wind-cheating shape. That could call for more extreme styling, like that of the Toyota FT-Bh concept, with its 0.235 Cd.
We've discussed the benefits of low weight more than once, but reaching the targets needed to make big strides in efficiency might call for more obvious changes than clever use of materials under the skin.
2011 Renault Captur Concept
If this sounds like bad news to fans of cosseting, luxurious interiors, then perhaps some of the benefits might draw you in. Not only will economy improve everywhere, with less mass to move around, but so will performance, braking and handling. Not only that, but with less interior trim and narrower seats, interior space would improve.
You might have to take a slight hit on equipment levels, but it's likely that much of the equipment that remains will be controlled by something like a smartphone, reducing the need for a complicated dashboard full of electronics.
There's no getting away from it--engines will have to change, and it's unlikely they'll be getting bigger and more powerful. In fact, power figures are likely to remain fairly static, while capacity decreases. Turbochargers and hybrid systems are likely to become more prevalent, as both can be used to prop up the power of downsized engines.
Paired with both reduced weight and better aerodynamics, the smaller engines will be able to deliver better economy, without struggling against the large, heavyweight bodies of modern cars.
We're also likely to see fewer cylinders, already a bone of contention for muscle car fans. That said, muscle cars are never likely to get 125mpg anyway, so it's really the smallest cars where cylinders will decrease.
Volkswagen Polo Blue GT
Small-capacity diesel engines are another option, but diesel is becoming scarce in small cars even in Europe, as they cost more to manufacture and the profit margins are becoming too slim.
One thing is for sure: Creating a 125 mpg car won't be easy, and it will require compromise. It goes without saying that some of the changes above may well add to the cost of a vehicle, particularly if a car is to achieve that sort of economy in the near future.
It's very likely that electricity will play a large part in hitting high figures. Theoretically, it's possible for cars like the 2012 Toyota Prius Plug-In and the 2012 Chevrolet Volt to hit 125 mpg on some journeys, by combining zero-emissions electric running with small amounts of fuel.
However, to sustain 125 mpg over longer distances, cars will need more than a token 40-odd miles of electric running, so some of the other factors above will play a major part.
And in the long term, it could all be academic, as battery electric vehicles make "miles per gallon" figures a moot point...