Crash test ratings are just as pernicious as the CAFE targets, as they evaluate how the vehicle fares in the crash, but do not factor in how well whatever the vehicle hit did. This results in bigger, heavier vehicles.
They also do not factor in the odds of getting in a crash in the first place. Thus, we get vehicles with worse and worse driver visibility due to thicker support columns and smaller windows.
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.
1reducing 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.
1If 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.
1Is increasing fuel efficiency actually a goal of the government? Given how the vast majority of roadway funding comes from gas taxes in most states, and the Federal government also gets a fair amount of revenue from them (while simultaneously the national debt, ever-unpopular, is growing at a rapid rate), it feels like increasing fuel efficiency is actually negatively-incentivized.
On that note: Are there any good proposals for how to replace that revenue source for when electric vehicles eventually eclipse ICE vehicles?
Doesn't the same general way work -- at least for charging at a station. I suppose a similar approach could be used for home charging by requiring the hook-up to have some meter applied and then report consumption by the meter -- or add the tax to the electric bill for collection.
But yes, it is a bit of a different setting for collecting tax revenues for electric vehicles. But might also just be one of the add-on fees public utilities already add to my bill to cover the infrastructure build and maintenance costs.
Perhaps vehicles where the front is tall should also be taxed, based on expected risk to pedestrians. Napkin math indicates values comparable to CAFE fines.
But then I'd need to understand why they switched to this system (if it's crash safety we should legislate that directly) and it's not clear that continued regulatory whiplash is worth it.
Because of competition from Japan, US carmakers have made virtually no profit from small vehicles for a long time. Larger vehicles are more profitable for various reasons, including: https://en.wikipedia.org/wiki/Chicken_tax
So, there was intense lobbying from US carmakers to not penalize larger vehicles, with the political cover being "not hurting small businesses and farmers that need trucks, or families that need to carry their kids".
Cars and trucks are getting bigger, and I had a vague sense that fuel economy regulations were partly to blame. Looking into it, it's hard to say how much is regulations vs people wanting to buy vehicles that look rugged, but the regulations really aren't helping.
This chart is the core of it:
This is what manufacturers were looking at when they decided to build today's cars. To figure out the target fuel economy for a vehicle you first calculate its "footprint", which is the area between the wheels. On our 2013 Honda Fit that's 4.8ft side-to-side and 8.2ft front-to-back, for a footprint of 39sqft. Then you ask if it's a car or truck. This tells you which curve to use, and where along it to look.
Looking at the chart we can now see why it's hard for Honda to sell a Fit today. The best Honda could do for a five-seater non-hybrid hatchback is maybe a CAFE rating of 44mpg. [1] This puts them 23mpg short, and if Honda was a one-model car company they'd expect to owe $3,910/vehicle in fines: $17 per 0.1mpg shortfall. Since the regulation is about an average across all the cars they sell the actual effect is both lower and more complex, and maybe something like $2k.
Aside: the fine structure here is a sad artifact of us thinking in miles-per-gallon instead of gallons-per-mile. Going from 25mpg (0.04 gpm) to 50mpg (0.02 gpm) saves as much gas as going from 50mpg (0.02 gpm) to infinite (0 gpm). But the penalty for being below a target is calculated on the gap in miles-per-gallon and not gallons-per-mile. If you miss a 50mpg (0.02gpm) target by hitting 25mpg (0.04gpm), or miss a 75mpg (0.013gpm) target by hitting 50mpg (0.02gpm), you pay the same fine even though the first involves burning much more counterfactual gas: over 10,000 miles the first burns 200 gallons more than its target while the second only burns 67 more.
What did Honda do? They discontinued the Fit, and replaced it with the HR-V. It's bigger and heavier, and looks like it was trying to be a "light truck". Combined with its larger footprint that would give a much lower target: 49mpg instead of 67mpg. It still doesn't hit that, but it's less of a penalty. And then it doesn't actually count as a light truck, though I don't know if that was the plan from the beginning or a compromise they had to accept.
Overall, this regulatory structure taxes manufacturers more for making small low vehicles, the kind that are easiest to make fuel efficient. Here's where I would write that this is counterproductive and we should stop, except we sort of already did. In 2025 the penalty for non-compliance was set to $0 as part of the OBBBA. This means in some sense manufacturers are free to make small cars and trucks with achievable mileage. Except the rest of the structure is still there, complete with the distorted incentives, and ready to be reinstated by a future government.
If at some point there's political will to improve this situation, and a carbon tax remains off the table, I'd like to see a return to the simpler Ford-era system where targets didn't scale with vehicle size. But then I'd need to understand why they switched to this system (if it's crash safety we should legislate that directly) and it's not clear that continued regulatory whiplash is worth it.
[1] The closest to 67mpg would be something like the first-gen Honda Insight. This got very close, but seating only two people with a lightweight construction that would do very poorly in modern crash testing. If you're willing to make it a hybrid, which does add significant cost, it is possible: the the Jazz e:HEV (essentially a hybrid Fourth-generation Fit) would probably come in around 72mpg.
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