Aside: the fine structure here is a sad artifact of us thinking in miles-per-gallon instead of gallons-per-mile.
I once helped a (very smart and competent) colleague build an excel model of the benefits of vehicle lightweighting. I had to explain to him that because he was applying a linear approximation to mpg instead of gpm, his model claimed that reducing the weight of a car all the way to 0kg would only save a third of its fuel consumption.
reducing the weight of a car all the way to 0kg would only save a third of its fuel consumption.
I actually think this is true, for a different reason.
Say you could make an F-150 truck with all of its components massless. It still has the same air resistance, which uses maybe 1⁄3 of the energy as trucks aren’t very aerodynamic. But to match the performance of the original truck, you need
wind stability (a massless truck just blows away)
cornering stability (all weight comes from passengers or the bed, which means a very high center of mass and therefore low stability)
towing (any tongue weight would flip the truck backwards)
All of these mean you need to add ballast. Suppose you add lead plates to the bottom of the truck totaling half the mass of the original. Then your energy consumption compared to the original is (1/3 from air resistance) + 0.5 * (2/3 from rolling resistance and braking) = 2⁄3 already! I’d guess this would still have significantly worse towing vs an F150 because getting enough friction to tow something uphill basically requires a minimum truck:trailer mass ratio.
With a smaller car towing isn’t a concern, but then you have safety issues with such a light car, so you’re probably still limited to half the mass of the original. To get better than 2⁄3 the fuel consumption of the original your massless components would need to magically provide downforce only when cornering or something.
If you had a perfectly massless car, at rest in perfectly still air, and you accelerated, would it go forwards (because the tyres are nevertheless still in contact with the ground albeit with 0 force pushing them into it)? Or would the wheels just spin (because since there’s a 0 term in the friction equation)? Or (let’s say it’s rear-wheel drive) would the wheel stay in-place and the car rotate around it, like a motorcycle doing a wheelie?
I would’t describe downforce only when cornering as “magical”: seems eminently achievable with active aero. A control-surface active aero system was patented by BMW in 2024 and a jet-assisted version was demonstrated in 2025.
If the car could be powered by a propeller (or for that matter a rocket..) we could simply vector the thrust to give us whatever balance of forces we want. Vector more upwards thrust to push the car into the road and give more grip, vector more downwind thrust to compensate for wind, vector all thrust dead astern for maximum acceleration, &c.
I once helped a (very smart and competent) colleague build an excel model of the benefits of vehicle lightweighting. I had to explain to him that because he was applying a linear approximation to mpg instead of gpm, his model claimed that reducing the weight of a car all the way to 0kg would only save a third of its fuel consumption.
I actually think this is true, for a different reason.
Say you could make an F-150 truck with all of its components massless. It still has the same air resistance, which uses maybe 1⁄3 of the energy as trucks aren’t very aerodynamic. But to match the performance of the original truck, you need
wind stability (a massless truck just blows away)
cornering stability (all weight comes from passengers or the bed, which means a very high center of mass and therefore low stability)
towing (any tongue weight would flip the truck backwards)
All of these mean you need to add ballast. Suppose you add lead plates to the bottom of the truck totaling half the mass of the original. Then your energy consumption compared to the original is (1/3 from air resistance) + 0.5 * (2/3 from rolling resistance and braking) = 2⁄3 already! I’d guess this would still have significantly worse towing vs an F150 because getting enough friction to tow something uphill basically requires a minimum truck:trailer mass ratio.
With a smaller car towing isn’t a concern, but then you have safety issues with such a light car, so you’re probably still limited to half the mass of the original. To get better than 2⁄3 the fuel consumption of the original your massless components would need to magically provide downforce only when cornering or something.
If you had a perfectly massless car, at rest in perfectly still air, and you accelerated, would it go forwards (because the tyres are nevertheless still in contact with the ground albeit with 0 force pushing them into it)? Or would the wheels just spin (because since there’s a 0 term in the friction equation)? Or (let’s say it’s rear-wheel drive) would the wheel stay in-place and the car rotate around it, like a motorcycle doing a wheelie?
I would’t describe downforce only when cornering as “magical”: seems eminently achievable with active aero. A control-surface active aero system was patented by BMW in 2024 and a jet-assisted version was demonstrated in 2025.
If the car could be powered by a propeller (or for that matter a rocket..) we could simply vector the thrust to give us whatever balance of forces we want. Vector more upwards thrust to push the car into the road and give more grip, vector more downwind thrust to compensate for wind, vector all thrust dead astern for maximum acceleration, &c.