There are many variables you didn't include. I understand you mentioned same size engine. For the car: Normal driving conditions? Shifting up and down? Constant RPM or varying RPM? Weight of car? Drafting behind other vehicles? A lot of drag because of roof racks or large car/truck body? What altitude are you driving at? What temperature is it? For the plane: Is it an ultralite? Does it have a fixed propeller or "constant speed" variable propeller? What altitude is it flying at? What direction and speed is the wind blowing? What temperature is it? All of these factors can drastically affect the outcome of your question/answer. Car engines and plane engines are not actually identical anyway. They may be close but planes run on 110 octane fuel and cars on 87. If generalising with all these factors, I'd have to say the plane has the upper hand. One of the main reasons I say this is the plane would have the luxury of placing the throttle at max economy RPM and flying at optimum altitude for best range (note that this would not necessarily be near stall speed). A car cannot just drive along with the engine at idle on the freeway. Additionally, car manufacturers just arbitrarily set the air/fuel ratio to a rather inefficient setting. In a plane, the pilot decides what that ratio is. Changing this ratio on a car is possible but would at the very least void the car's warranty. :-) I've linked an interesting article about a new type of spark plug (I know it looks a little conspiracy theorish but it seems factually sound) that allows a drastic change but overall, I'd say the plane has the upper hand by a large margin. If you don't like having all the variables in the picture, just make your calculations from the weight of the vehicle. Again, the plane will win if we're talking about a small SEL plane. I hope this answers your question :-) http://www.rexresearch.com/krupa/firstorm.htm http://www.robertstanley.biz/firestorm.htm

Te car will get better efficiency. Especially with conventional asperation. A gas engine loses about 6% of it's power for every 1000 feet over 4000. . The plane would cruise at over 10,000 so expect a loss of 36% power as well as fuel efficiency right tere. And the power from the engine to a car's wheels is more direct than a prop churning air. But can a plane have better efficiency than a car. Yer. Such as electric planes vs a gas car. Just not with te same engine. Your straight road killed the competition too. Had the plane gone straight and the road windy and indirect, I might have favored the plane. Use a car engine in an aircraft and it gets an experimental license. I belive that ends up with tearing the engine down and magnaflux inspection every 150 hours or less. That sure would offset any fuel advantage.

Airplanes actually get better efficiency at higher altitudes due to a lessening of drag, up to a point. 8000 feet is often used as a point of reference. For real-world comparisons, I flew to Cedar Point (a great roller-coaster park) from Chattanooga, Tennessee in three hours, covering slightly over 400 nautical miles (around 450 standard miles). Driving to the same location, in what seems to be a fairly straight trip up I-75, puts about 575 miles on the automobile. I burned just a hair under 30 gallons of 100LL (100 octane, low-lead) in the plane, for about 15 standard miles per gallon. If I try to equate it to the distance I would have had to drive, that gives me 575/30 or a little over 19 mpg. My honda gets 29 mpg, so the car is more fuel efficient by mileage. A better question would be, 'Is it more COST efficient?'. If you include two meals for the 12 hours driving up, another two driving back, and an additional two days in hotel rooms, it starts getting fairly close. This doesn't allow for wear-and-tear on either vehicle, but airplane engines are assumed to be near need of an expensive overhaul at every 2000 hours of operation, depending on the engine and how it is maintained. some more, some less. My company reimburses auto mileage at a rate around 55 cents per mile, so there would be some substantial deduction there as well. As far as the engine discussion, it takes a lot more continuous horsepower to keep an airplane flying than it does to keep a car moving down the road at 65 mph. Economy cruise is usually 55-60% throttle, standard cruise at 75% (~155 mph in a Piper Arrow). Run a car for three hours straight at 75% of redline, and I would think that your mileage would greatly suffer, if the engine held together. For my honda, that would be between 4500 and 5000 RPM. Being a solid vehicle, it would probably hold together, but I'm sure it would shorten it's life. Your airplane's advantage is that it takes what would be a 4-day trip and turns it into a 2-day trip. Or what would be a full weekend away from home becomes only one day. It makes things possible where they were not before. Not to mention that it's fun and pleasant to go on a long trip without having to dodge large trucks and traffic jams. Electric planes? They made one that (I think) has about 30 minutes of air time, which is right at the minimum required to have left when you land (by the FAA). Batteries weigh too much for the power contained in them, so are very poor choices for airplanes, where weight is at a premium. Perhaps when these fuel cells get further along, it will be possible to run a plane on a hydrogen fuel cell. I certainly hope that something better is created, until then we will continue flying on leaded gasoline.