Our DIY Car: The Quest for 100 MPG

Enlarge Image Image Gallery 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. KATHERINE LOECK

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Last year we introduced MAX, 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 100 miles per gallon 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 Race, 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.

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