MAX: Our Quest for a DIY 100-MPG Car

Progress continues on MAX, the home-built, 100-mpg car (prospectively) that you’ll be able to make for $10,000.

100-mpg car

MAX is a sports car powered by a Kubota diesel engine that’s normally used in tractors. After MAX has been perfected, we will have plans available for those who want to build their own.


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Last year we introduced MAX, the MOTHER EARTH NEWS contender in the Auto X Prize fuel-efficient car competition (see Here Comes the 100-mpg Car). For those who may have just tuned in, the Progressive Insurance Automotive X Prize is a $10 million competition to create cars capable of achieving 100 miles per gallon in fuel economy. Entries must have a design that would allow for the production of 10,000 cars per year at the price most appealing to the manufacturer’s stockholders. We’re aiming for a more affordable approach: a DIY, fuel-efficient car that you can build for about $10,000.

Part of what I want to prove with MAX is that if I can build a 100-mpg car using off-the-shelf technology, the major automakers should be able to do the same, if not even better. Sure, MAX is different than a family sedan, but those car companies also have different resources than we do here in the Kinetic Vehicles garage. After MAX has been perfected — whether we hit 100 mpg or not — we will have plans available for those who want to build their own.

MAX is a sports car powered by a Kubota diesel engine that’s normally used in tractors. The car has turned heads and drawn crowds everywhere we’ve gone, and shown its heels to far zootier vehicles in everything from “play dates” on the twisty roads of southern Oregon to formal endurance racing from Berkeley, Calif., to Las Vegas (see MAX Wins 800-mile Road Rally, Without Gas).

You may be wondering whether MAX is practical. Well, we have more than 7,000 miles on its wheels now — everything from grocery getting to freeway flying — and it has served us reliably. For fuel, we’ve used everything from jet fuel to olive oil.

I bet I know your next question: Is MAX getting 100 mpg? Not yet. But so far we have the chassis and power source sorted out, so now we can improve MAX’s body, which will really improve its mpg.

Building a Better Body

The only thing holding us back from 100 mpg is air resistance. MAX’s current body looks cool and classic, but it’s an aerodynamic disaster. We’ll never see 100 mpg with this design. Frankly, it’s a bit amazing we’ve been nipping at 70 mpg (which we’ve achieved when cruising slowly). But we know what we need to do: Make a new body that has half the drag of the current one.

For an object to pass smoothly through air, it must gently nudge the air out of the way in the front and then allow the air to collect together again in the back after the object has passed. The ideal object is teardrop-shaped — rounded in front and pointed in back, with a smooth transition along the sides so the air slips closely around the object. Aerodynamicists compare the air resistance efficiency of objects by their drag coefficient (Cd), which is the drag the object produces compared with the drag of a theoretical flat plate the same size as the object traveling flat side against the wind.

Most objects have a drag coefficient of less than one. If you round its sharp edges a bit, even a shoebox has a drag coefficient of less than one (0.7), because the top and sides of the box keep the air better organized than if it were just stumbling over the end of the box. Some cars have a drag coefficient to be proud of, such as the 2010 Toyota Prius with its 0.25 Cd. In other words, the Prius has one-fourth the drag of a flat plate the same size as a Prius.

MAX, like the Lotus Seven sports car from the ’60s that inspired its initial style, currently has a 0.7 Cd, which means you could literally put MAX in a snug shipping crate (or a giant shoebox) without making its air resistance any worse. This body has served MAX well — it gave us a year of street experience while many other Auto X Prize contenders hadn’t left the drawing board — but it also has major deficiencies.

For starters, the air has to go over too many things twice. The front wheels and fenders go through the air, the air closes in behind them, and then the rear wheels and fenders go through the same air again. MAX’s new body will be enclosed between the two sets of wheels so that the air only has to pass that way once.

Next, there’s too much stuff sticking out in the wind. We’re stylin’ with our 7-inch headlights, but we might as well bolt a couple of porridge pots to the fenders. The headlights will go undercover in MAX’s new body.

Third, the windshield is an air dam. We’re replacing it with a steeply angled split windshield because we can’t find a curved safety glass windshield small enough to fit.

Last, the back of the body just ... stops. There’s nothing to coax the air back together after MAX has gone by. MAX makes a big hole in the air, and that vacuum tries to suck MAX back into it. We need to work on that.

The rules for the Auto X Prize competition aren’t official yet, but we’ve been given some guidelines. For starters, no open cars — so MAX is getting a removable hardtop. It’ll be a hardtop because, with our skills and tools, we can make a fiberglass top cheaper and easier than we can sew a cloth top. It’ll be removable because there are too many days that are just too pretty to stay indoors.

We can’t look to open-top racers such as the Lola Mk1 anymore, but the race cars of the mid-20th century are still a good source of inspiration. By the mid-1950s, automobile streamlining was pretty well figured out, and using classic racers as our styling guide justifies MAX’s total lack of modern conveniences. If a car looks like it won its class at the 24 Hours of Le Mans race (circa 1954), nobody expects it to have cup holders, electric windows, and other such luxuries. Every component not present saves weight and money, so we’re keeping MAX as simple as possible. Besides, those of you who build your own MAX can add all kinds of custom features!

100 MPG on a Budget

We are confident we can re-design MAX to cut its aerodynamic drag in half. But how hard will this hit our budget? Our challenge is to figure out how cheap we can make a body that will do the job.

MAX is approaching the ceiling of our budget with its $4,600 engine, $1,000 roadster body, and $3,000 in other materials and parts. Can we make a full, streamlined body for another $1,000?

If we can, I think it will be worth the money. I’ve driven MAX in rain, snow and hail, and while my first impression was that it was no tougher than driving a motorcycle in similar weather, motorcycles don’t fill up with slush, whereas MAX does. So the guidelines say roof and doors, and that’s fine with me. Here in Oregon, a roof and doors will double the number of days that MAX can be driven.

On the other hand, some of the other Auto X Prize guidelines seem contrary to the competition’s ultimate goal. One guideline tells us we’ll need engine modifications to get quicker acceleration.

Right now MAX can go from zero to 60 mph in 20 seconds. The Auto X Prize folks are insisting all entries hit 60 mph in less than 18 seconds. So we’re experimenting with all kinds of things I don’t want DIYers to have to fool with, such as propane injection and bumping up the turbo boost, resetting the engine to be able to run from 3,000 rpm to 3,600 rpm. We’re going for the minimum horsepower boost that will keep MAX qualified for the competition. And, of course, we don’t want to do anything that will increase fuel consumption too much. But the reality is that more power will require more fuel, so I’m disappointed to see performance get such high billing in a fuel economy contest.

Let’s get back to building the body. We didn’t know which type of body we would use when we started the project, so we geared ourselves up to ride off madly in all directions. We built the Foam Ranger, a machine that takes a Rhino file (a 3-D design program), runs it through RhinoCAM (a computer-aided-machining program), and uses the output to guide three motors and an air grinder in carving the shape that was in the Rhino file out of a block of Styrofoam. It makes a lot of noise and a spectacular cloud of Styrofoam chips (which we recycle at a local insulation company). When it’s done carving, we have a full-size body part, which we cover with fiberglass and epoxy to give it a hard surface.

If the part works the way we want, we will make a mold from it and make more of the part. This is 50 times faster than the traditional sculpting technique (which dates back to Michelangelo: take a big rock and knock off everything that doesn’t look like David).

Best of all, the software lets us take advantage of Rhino experts and car designers all over the world. This is particularly valuable to people like me, who have lots of great ideas but lack the talent to execute them. When I show people my car drawings, they say, “Oh, that’s darling. I didn’t know you have grandchildren.” But when folks see the Foam Ranger whittling away, they think I’m a genius. The truth is other DIYers have walked this path before, and home-built, computer-controlled machines are fairly common — this one’s just a lot bigger than most. But don’t tell anybody, ’cause everybody around here thinks I’m brilliant.

The trick to keeping the body as inexpensive as possible will be to keep the complicated bits to a minimum. The simple parts of MAX’s new body — such as the hood and sides — will be made from pre-painted roofing metal, which is light, easy to work with, and conforms well to simple curves. The more complex parts with curves, such as the nose and the fenders, will be made out of fiberglass.

The next trick will be to make the fewest and smallest complex parts that hold together the big, simple parts — and still achieve 100 mpg, all at a price we DIYers can afford. To quote Albert Einstein, “Everything should be made as simple as possible, but no simpler.”

Stay tuned. You’ll see more about MAX in these pages. In the meantime, you can follow our progress online via the MAX Updates.

MAX Will ‘Escape’ Again!

The second annual Escape from Berkeley alternative fuel rally runs Oct. 3 to Oct. 6, 2009, from Berkeley, Calif., to the U.S.-Mexico border. MAX won the inaugural event (Berkeley to Las Vegas in 2008), and will be on hand this year to defend the title.

The event takes a whimsical look at a none-too-whimsical subject: How would people travel if the petroleum and financial industries hit rock bottom at the same time? The answer is half Jules Verne and half Back to the Future, with costumed steampunks begging for biofuel along the way — everything from scrap lumber to fryer oil. The vehicles range from scrap-yard science experiments to projects on the university level, and it’s nearly as fun to watch the event as it is to compete.

Competing, however, pays better. Along with petroleum-free bragging rights, first place wins $10,000. The rules have changed a bit this year (no bartering, and the fuel has to be waste products), so last year’s winning strategy (we swapped copies of MOTHER EARTH NEWS and other memorabilia for cooking oil) won’t work. But that won’t stop MAX.

Breaking News

As we went to press, we decided to trade our Auto X Prize entry form for a press pass. In short, the mass production requirements of the competition were interfering with our DIY goals. We want to give MAX our full focus as a car you can build. When the Auto X Prize races begin, Jack will report on the competition as he drives MAX along the way.

To hear more about this decision, read MAX Update No. 32: Why We Resigned from the Auto X Prize. To keep tabs on all things MAX, visit the 100-mpg Car page.

jack mccornack
10/18/2009 9:09:55 PM

KeithOliver noted that... > ...anyone trying to copy the MAX concept is going to have to meet emissions regulations in effect at the time that they register. ...and that's true, but there are 50 different regs in 50 different states, and in most states homebuilt cars don't have to meet the same standards as high volume cars. > MAX was started a Loooong time ago... No it wasn't. MAX was conceved in '06, we started collecting parts and fitting things together in '07, and its registration date was '08, which is when MAX first hit the road. It's not registered as a heavily modified older car, it's registered as brand new in 2008. ...prior to soot filters and selective catalytic reduction being required... There are manufacturers working hard on those problems. Some of these manufacturers were quite interested in MAX as a demo platform before the final round of the Automotive X Prize rules were released, and affordable ceramic soot filters are just around the corner, and urea injection is coming out of the labs and hitting the roads already. It's not insurmountable. ...but that will no longer be the case in 2010 and beyond. One big thing MAX has in its favor is it doesn't make much exhaust per mile. I'm not sure how the other American auto manufacturers are doing it, but if they can pass EPA muster burning three times the fuel that MAX burns, our EPA-certified-for-agriculture engine might pass the auto emissions requirements, just the way it is.

jack mccornack
10/18/2009 8:33:05 PM

Hi Bob, > I really feel let down that you are giving up the X-Prize contest. :-) You and me both, man. > The reason that you give that you would have to modify the engine to go from 20 seconds for 60 mmph to 18 seconds doesn't make sense to me. I know I've written a lot of words on the subject, but don't recall writing those. I'll acknowledge that the 0 to 60 performance issue was one of the first clues that the X Prize folks and I weren't seeing eye to eye, but it's not why we withdrew. I could write reams of details, but the basics are in MAX Update #32 in the MAX Update blogs. Improving 0 to 60 times by 10% isn't trivial, and I don't think the body change will do it. The streamlined body will add some weight, and it's possible the will hurt low speed acceleration as much as the streamlining helps it. We can meet their acceleration requirements with engine mods, but then we're getting out of our knowledge base re emissions, and I know the Kubota engineers worked hard to balance power against pollution. And of course, we could have got with the X Prize program and converted to electric, but then the budget would have skyrocketed, as would our carbon footprint. So we're keeping MAX a DIY car and paying more attention to economy (both fuel economy and construction/purchase price economy) than to performance.

10/17/2009 12:11:20 PM

Unless I'm mistaken, there is 1 big problem with the concept, and that is that anyone trying to copy the MAX concept is going to have to meet emissions regulations in effect at the time that they register. MAX was started a Loooong time ago, prior to soot filters and selective catalytic reduction being required, but that will no longer be the case in 2010 and beyond. I think the only chance the rest of us have is to retrofit an existing vehicle with a registration date prior to 2005. The earlier, the better, since the less stringent the emissions regulations. I have been looking and one cabn find Honda Insights with dead batteries for $1-2k. Good areodynamics come right out the box.

jim price
10/16/2009 1:25:11 PM

Yo! Love your project. Big trucks use a two speed rear-end to get 12/16 forward gears; Electric actuated add-on overdrive units are available for cars. With proper transmission/ rear end gear selection your car would be a real hotrod around town and triple overdrive would help your quest for 100 mpg. One of the ultra high performance cars uses an eight speed tranny but their goal is 200+ mph. Good luck,--------jim------

10/16/2009 9:06:55 AM

I really feel let down that you are giving up the X-Prize contest. The reason that you give that you would have to modify the engine to go from 20 seconds for 60 mmph to 18 seconds doesn't make sense to me. Just by adding a streamlined body will give you at least 2 seconds.

jack mccornack
9/30/2009 8:01:22 PM

Interesting points, Kit. Over-the-highway trucks tend to have a higher ratio of rolling resistance vs aerodynamic drag than cars, because they typically have only triple the frontal area of a car but 20 times the loaded weight. They need a lot of power to deal with the needs of acceleration and hill climbs, but once you've got them rolling, they roll pretty takes surprising little to keep them going on flat ground. So as far as fuel economy goes, trucks don't suffer from poor aero as much as cars do. That said, if a trucker can add an air deflector to the top of the cab and go from, say, 6 mpg to 6.1 mpg, and if you drive 100,000 miles a year, that's 275 gallons a year. Over a 20 year service life, that's 5500 gallons.

jack mccornack
9/30/2009 7:35:09 PM

t. brandt, reducing drag does have its limits, but we're nowhere near them yet. For mileage bang-for-the-buck you can't beat small motorbikes, but it is possible to equal the drag (and mileage) of a typical motor scooter with a light and streamlined car. Craig Henderson broke 100 mpg in his Avion a quarter century ago and last October hit a personal best of 113 mpg cruising from Canada to Oregon. Streamlining motorbikes works wonders too. If you scroll down to MAX Update #25 you can link to the Craig Vetter Fuel Economy Contests, which topped out at 470 mpg for a street legal motorcycle at highway speed. Craig (Henderson) hopes to have Avion kits available in the spring, he just finished production body molds for the kit and his target is a car you can build for $30,000. There's no question that 100 mpg cars are possible (the Avion web site is, the question we're trying to answer with MAX is, can you build one for ten grand?

9/30/2009 11:33:12 AM

Love the article guys! However air drag may not be so much of a problem as you think. Consider Schneider Trucking,SLOW,under powered,Cabin overs,aerodynamics of a brick bat and an absolute traffic hazard,get all fuzzy with their 7.5 mpg.! A Freightliner Columbia will get about 5.5-6.5 mpg. Outwardly the Columbia appears more Aerodynamic.Then there is a good friend that has a 1998 Freightliner Classic Condo.550+ to the drivers hp. It is an Aerodynamic train wreck that is almost never driven under 70 mph. His books will show a 7.2 mpg fuel average.Unlike Schneider he stays loaded to the gross,80,000 lbs. almost constantly and does idle his truck when and where he can for either heat or ac. Differences are he put an after market larger turbo on,changed the injector clearances and timing,has a 13 spd. trans.,keeps the engine in its "sweet spot" better,and under the hood the air is dealt with more efficiently than the Columbia. I put a hydrogen generator on a VW Jetta diesel and got 70 return with no other alterations. I really feel your 100 mpg is doable with some more out of the box thinking! Good Luck!

t brandt
9/22/2009 4:06:34 PM

The 100mpg auto is really a pipe dream. There already is a 100mpg vehicle: the 50cc motor scooter. they've been around for 60 years. Some hi school physics: E = 1/2mv^2 + drag + heat. Reducing drag has it's limits. Nature has already figured out the best shape: the fish & the bird body. Unfortunately, the internal combustion engine is inherently inefficient and 90% of energy released in burning fuel is lost as heat. The only thing we can really do is minimize the "m" or the "v" in the equation. That leaves us with the tiny, slow scooter.

jack mccornack
9/19/2009 5:16:34 PM

Thank you, Sam. Your ideas aren't silly at all, but they need to be tempered by our $10,000 budget. Lightweight wheels clutch brakes etc would save fuel (lightweight --anything-- saves fuel, rotating parts in particular) but as far as bang-for-the-buck goes, I think streamlining will give us the best return for our money...hopefully we'll have enough left over to change the trans and axle lubes to synthetic. And Robert, while every state has different rules for homebuilt car registration (some easy, some tough), my personal MAX, which is registered at my home in Oregon, is legal to drive in any state, just like any other car with an Oregon plate. If you register your own homebuilt car in your own home state, the other 49 states will honor your registration as well.

9/18/2009 8:42:21 PM

Do we know if this thing is legal in any state ?

sam strohl
9/18/2009 9:35:49 AM

let me start out by saying i love the max,great idea.if you can build this in your garage and get it to 100 mpg that would be totally awsome! love your new body design,can't wait to see this on the car! this might seem like a silly idea,but i think you could also improve your milage by using aluminum wheels and better tires with less roll resistance.what about syntheyic lubricants? dont know what you have for a driveline but there are several areas this can be improved.lightwieght cluch,driveshaft, brakes and rotors to name a few.anyway keep up the good work.