The Kia Rio supposedly carries five passengers. By reducing the passenger capacity to just one, that is 80 percent fewer passengers, you might reduce the vehicle weight by 80 percent also, without using any special materials. However, one might want to use lighter materials to reduce the weight further so that more safety features can be added. The net result should be the 70 percent weight reduction to achieve renewability for the fuel supply.

5-passenger car weight = 2403 lbs.
1-passenger car weight = 481 lbs.

5-passenger car fuel economy = 30 mpg.
1-passenger car fuel economy = 150 mpg.

The 1-passenger version would still have standard steel construction. No aluminum or composite components necessary. But aluminum has up to 8 times the specific strength of steel, depending on the alloy used. So the vehicle weight could be reduced further. Some parts might be the same on both the heavy and light vehicles, such as wheels, lights, fasteners, trimming, etc. But I doubt it would be hard to make a car light enough to pick at least one end up off the ground by hand, say, a few hundred pounds.

Ground clearance should be a piece of cake. It's safety that's going to be a challenge. Airbags, and lots of 'em. All around the vehicle. And maybe spars that transfer loads from deformed airbags to ones that aren't directly impacted, absorbing even more energy in an accident.

An interesting model would be a hamster ball, but one that flexes easily. Assuming it's rolling along at 60 mph. Could it impact another similar hamster ball and protect its passenger?