New Gas Mileage Rules Will Reshape What Americans Drive: Aerodynamics And Weight

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2014 Mercedes-Benz CLA 45 AMG, First Drive, Bilster Berg

2014 Mercedes-Benz CLA 45 AMG, First Drive, Bilster Berg

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No, we won't all be driving golf carts.

In fact, as cars change over the next 12 years to meet much tougher fuel-economy rules, automakers will likely do their best to keep the changes invisible.

Your next vehicles will deliver better gas mileage than ever before, no matter how large or small they are.

By 2025, the industry has agreed to meet an overall corporate average fuel economy (CAFE) level of 54.5 mpg.

That translates to about a 42-mpg combined EPA rating on the window sticker, due to differences in how fuel efficiency is calculated.

That's lower than three different Toyota Prius hybrid models achieve today, but the average includes everything from minicars to full-size pickup trucks.

So what are the technologies that will let automakers comply with the gas-mileage rules--and how will they change your next car?

We've broken this rundown into three parts:

Today, we'll look at the increasingly slippery shapes cars will take on--many of which you probably won't notice--and the diets they'll be put on to lose excess weight.

In both cases, the changes may not seem as radical as you imagine.


One of the easiest ways to save fuel is to make a car smoother and slipperier above 40 mph, when the energy required to overcome wind resistance starts to rise.

Every new vehicle, from sleek supercars to bluff, blocky-looking pickup trucks, now spends hundreds or thousands of hours in the wind tunnel.

Body designers and aerodynamicists refine every element, from wing mirrors to small "spats" at the corners of the wheel arches, to reduce the airflow turbulence that causes drag.

Wind tunnel testing the MINI Cooper John Cooper Works GP

Wind tunnel testing the MINI Cooper John Cooper Works GP

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Airflow is key over 40 mph

By guiding the air smoothly over and around the car, reducing the swirls and eddies where air gets trapped, less energy is required to push the car through the air.

A byproduct is that the car can also become quieter, with less wind noise apparent inside.

One way you can confirm just how aerodynamic a new car is: Take it on the freeway and open one window--say the driver's--and then another one on the other side, perhaps the right rear.

In many cars, the imbalance in airflow on the two sides produces a fast, unpleasant drumming or buffeting sound.

That old tip about opening your windows to save fuel by keeping the ventilation off?

Definitely no longer accurate; efficient aerodynamics save more fuel at speed than the newest electric air-condioning compressors use.

2011 Chevrolet Cruze Eco

2011 Chevrolet Cruze Eco

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Grille shutters, undertrays

Two simple, invisible ways that automakers can smooth out the airflow over new cars are adding active grille shutters and smooth undertrays.

First launched on the 2011 Chevrolet Cruze Eco, active grille shutters sit behind the grille and close off the airflow through the radiator when it's not needed for engine cooling.

That cuts turbulence (that air has to escape from the engine compartment somewhere) by simply routing the airflow around the car's nose and over the rest of the body.

GM now uses those active grille shutters on several different Buick and Chevrolet models, and Chrysler has added them to its latest 2013 Ram1500 pickup truck for the same reason.

As for smooth undertrays, they're simple, durable composite panels that cover the various components hanging underneath the car's floorpan.

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Comments (26)
  1. Great series of articles!

  2. Driver's will notice the difference with a lighter car when conditions are windy. I drive a light weight car and always need to drive with a "full" tank of gasoline in order to avoid getting jostled around.

  3. Buy a large BEV or PHEV/EREV and it will lower your center of gravity and increase the weight while giving you great efficiency...

  4. I have the same problem. My car weighs about 1.5 tons, but the wind this time of year is unbearable at highway speed

  5. Buy a PHEV/EREV and you will be approaching 2 Ton. :)

    Or buy a Tesla S and you will be well over 2 tons in weight and much lower center of gravity. :)

  6. It's not really about vehicle weight, but a ratio of aero properties to inertia. I've driven a 400 lbs solar car at freeway speeds in strong crosswinds. Even when the crosswind was interrupted by a passing big rig the effect felt behind the wheel was minimal. Think of standing on the beach (where I am right now) :-) when a wave hits you. It may knock you over and push you back if you're standing straight but if you dive into it you'll go right through it.

  7. Giving the looks of Tesla and Prius, I would think most people would choose Tesla while both of them are fairly good in area of drag. So, we need to reverse this trend of "boring looks" of hybrids that started with hybrids such as Prius.

  8. Omitted is the effect of EPA's new "Vehicle Footprint" requirements with 2017 model year.
    Expect mid & compact vehicles to get taller in future to maintain similar interior volumes as today. My thinking is EPA tests are measure MPG at lower speeds on average (less aero-drag), so will achieve a high EPA rating for compact high-volume vehicle; but real-world MPG measurements will much worse.

    We could also see OEMs making lots of 40-42 sq. ft. vehicles, and greater than 53 sq. ft. vehicles, but fewer with footprints in between. This is a reflection of CO2 g/mile rules being constant above and below these "footprints".

    In comparison, EPA doesn't directly incentitve a vehicles weight.

  9. Very good article - aero is under appreciated; and actually is important as slow as 28-30MPH, where it is about 50% of the total load on the drivetrain. 40-45MPH is a "sweet spot" for many cars in terms of MPG, given gearing and drag and how ICE's work.

    EV's are so efficient at almost any speed, and when you have a very low drag car - you can coast farther, and EV's don't idle, so they can gain even more from low drag.

    I take issue with your assessment of the low drag cars we have seen - some comes down to what we are used to, and some comes from odd styling choices. The GM Impact looked much better than the EV1. Narrowing the rear track can lower drag by as much as 40% overall which is HUGE, and a Cd under ~0.24 will require it.

  10. Low aero drag cars will need ride leveling - the angles at the back of the car are critical. Lowering a car too far though will *increase the drag* because the air gets trapped underneath and it can't flow freely. A smooth underside is much better than a chin spoiler.

    Smooth and flat wheels and tight wheel openings are also key to low drag. These contribute 20-30% of the overall drag, so we need to be open minded about rear wheel skirts, too.

    Many people could be driving EV's, which would about quadruple the energy efficiency of those people's cars. ICE's and hybrids @ X-Prize got ~75MPGe - EV's got ~138MPGe.

    Low aero drag will let EV's with today's batteries to have ranges 100-150+ miles - the same batteries that go 70-85 miles today.

  11. Personally, I like the looks of fender skirts on a car, but I know some won't like them due to snow and ice buildup in harsh weather. Some other vehicles {ex: Basjoos} with front and rear wheel skirting will be a tougher sell to the general public. I've watched your work with interest and I have a great deal of respect for your understanding of aerodynamics.

  12. Neil, I'll agree with Mark here, your knowledge of aerodynamics is helpful here for many of us. I will note that the few vehicles I've seen with fender skirts (or spats) haven't really impressed me visually, but given the clear advantages, I'll try to keep an open mind.

    Mark, can you cite a vehicle or two with fender skirts that you think look good just so I can take a closer look at them? The Honda Insight and EV1 are the two that come to mind but aren't personal favorites, personally, unfortunately.

  13. This series will omit something that for some reason Green Car Reports sidesteps: The EPA's power to regulate greenhouse gases by through mobile CO2 regulation, a.k.a. the tailpipe rule.

    GCR omission: active aerodynamics, engine stop/start technology and more are incentivized with CO2 credits. This is a large reason why they're coming. Automakers need CO2 credits.

    Another GCR omission: Starting in 2017, an automaker that violates fleet average CO2 limit 4 years in a row can have their highest CO2 emitting vehicles stripped of their "certificate of conformance" making it illegal to enter those vehicles into commerce in the Unites States. Read more here:
    1 -
    2 -
    3 -

  14. Nice article, the lightweight vehicles would still have to pass the FMVSS crash tests unless they are classified as motorcycles.

  15. Open wheel design is supposed to supersede todays cars, from what I have heard.

  16. The 56 mpg is easy to get. Just get the EPA to allow us to drive diesel cars. The 2008 Dodge Avenger with a turbo diesel gets 56 mpg, however, only the rest of the world can buy it. The 2008 Chrysler Town and Country turbo diesel gets 40 mpg. It is our own government that bans these vehicles from us. Questions, google the cars on the UK Chrysler web sites.

  17. @Pete: Remember that (a) UK gallons contain 20 percent more liquid than US gallons, so all MPG figures have to be reduced accordingly; and (b) the European fuel efficiency tests produce figures that are 10 to 20 percent more optimistic than EPA figures for the same car.

    So, your "56-mpg" Avenger is a 36- to 40-mpg car in EPA window sticker terms. Better than the gasoline versions, absolutely--but not as MUCH better as you think. FYI.

  18. Amusing... As in how the EPA somehow has banned the Town & Country and Avenger diesels while allowing a tremendous number of other new diesel vehicles to start selling.

    If Chrysler wants to sell diesel cars, the only thing stopping it is passing the same emissions tests that every other OEM with an interest in diesel already does or will. Instead, let's blame the government, that convenient boogeyman because Chrysler isn't technically up to speed or just doesn't want to spend the money to get approved.

    And even worse, somehow who wants to attack the EPA, yet apparently has no clue how ridiculous European mileage numbers are when compared directly to American ones. Prius: 71 MPG in Europe, 51 here, or 40% higher in Europe.

  19. Yes...lets blame the government for choosing arbitrary emission goals that make diesels very expensive to meet EPA rules. Diesels are not a slam dunk because of public perception that is some 40 years out of date. Why take development risk for something that no one might want anyway? I love this quote: "By 2025, the industry has agreed to meet an overall corporate average fuel economy (CAFE) level of 54.5 mpg." Really, John Voelker...when did the government care enough to gather a consensus of private institutions when it doesn't have to? The government is FORCE. The government is a real boogeyman. That is why people should be skeptical of everything it does. Go live in Venezuela for a while and see if you come back as a statist.

  20. @Randall Ah, Randall, there you are again.

    First, recall that most of the auto companies, the California Air Resources Board, the EPA, and the NHTSA jointly agreed on these standards--under the threat of even more radical action from CARB, which has a legally determined right to set its own standards. That was not in the interests of automakers. The Obama Administration got everyone together in a room & the horsetrading took place.

    Second, I'm curious about what you would propose instead. No emissions regulations?

    Third, note that 54.5 mpg CAFE ratings translate to about 42 mpg on window stickers.

    Fourth and finally, do me the courtesy of at least trying to spell my name correctly.

  21. You said it: "jointly agreed on these standards--under the THREAT of even more radical action from CARB" You must have believed all of those trips to the principal's office were meetings where negotiations for rules and appropriate punishments took place. CARB is the de facto regulator in the US. Car makers only sell cars in the US/Canada that will sell in California.

  22. At what point is enough regulation enough? Government never stops. What is to be gained by cleaning new car exhaust emissions more than already? The technology for reducing emissions has reached a practical limit. Further development of technology to reduce emissions will follow the Law of Diminishing Returns. The exhaust of old cars is the primary cause of smog, and as long as they are on the road, the air will be dirty.

  23. And by squeezing automakers, the government squeezes the population. I don't want my ability to buy a pick-up to be reduced by impractical requirements. If I need to drive somewhere, I'll use my car. If I need to haul something, I want to be able to buy a pick-up that doesn't cost six figures because of impractical regulations passed by the nitwits at CARB.

  24. There are more comments in this thread
  25. I commend you folks on this series of articles. It puts into perspective the game-changing approaches that the lethargic automotive industry will have to employ to meet the more-stringent CAFE standards. Sadly, it appears as if government regulations are the only thing that motivates automakers to change.

  26. Check website for a built in calculator to judge how hard it will be for aerodynamics to get the CAFE mpg on highway mpg. It is obvious to us at AERO18 that our patented front end will be needed for the radiator on cars and light trucks soon.

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