Why do piston engines in aircraft burn fuel at a higher rate than a comparable car engine? I've never understood why piston powered plane engines burn such a large  amount of fuel per time period than car engines. 

Can anyone elaborate on this?

I know that a C172M will burn ~5-9+ gph at 2200-2550 RPM between 2k-12k feet based on the POH.  This gives a decent idea of the fuel burn in a very narrow RPM range at various altitudes.  According to the POH, she came with a Lycoming 0-320-E2D. Wikipedia states that the 0-320 provides 150-160HP using a 5.23l engine.

My car on the other hand has a 2l 4 cylinder engine. At 70mph in top gear @ ~2300rpm she gets about 30mpg for a calculated 2.33gph. To me highway speeds at constant RPM provides comparable engine conditions to that of cruise phase of flight.

5.23l / 2l = 2.61 

2.61* 2.33mpg = ~6gph 

Does that basic analysis hold? Is the larger displacement the main factor in fuel consumption difference between the two? I consider this figure of ~6gph close enough to the 5-10gph listed in the POH for such a back of the napkin calculation if the analysis is more or less correct.

This leads to a follow up question: Is there a reason for the large displacement and low HP? For comparison a 5.7l truck engine from the 70s output 250HP for a gain of 100HP for .47l additional displacement when compared to the Lycoming. I'm in no way very knowledgeable about engine's but am guessing the answer lies in engineering trade offs for making the engine lighter or out of a different material for aviation use or for higher compression ratios for the fuel. Any detailed input would be appreciated. 

I know that Volkswagen and Subaru engines have been used for aircraft.  Although my research has been very light, it still looks like the fuel consumption is in the same range.

Going back to my car vs the C172M, or the EcoTec vs Lycoming, 173hp@ 5500rpm for the EcoTec and assuming a (most probably false) linear fuel consumption based on RPM,  

5500rpm /2300rpm = 2.39

2.39 * 2.33gph (@2300rpm) = 5.56gph

for comparable HP.

If my analysis provided here is close to the true answer, then I think I've cleared up my misunderstanding by writing out this question.  However, I would like someone more knoowledgeable about such things (possibly an A&amp;P who is also a gearhead) to pipe in and verify or enlighten my analysis.

Thanks!
As others have noted, airplane engines and car engines have very different duty cycles.  An airplane engine will typically run at full power for a few minutes during takeoff &amp; climb, then at a large fraction of max during level flight, which may last for hours, and be throttled back only during descent and landing.

An auto engine, by contrast, only operates at full power for a few seconds, as you for instance accelerate up that freeway on-ramp, then it will be throttled back to produce the small fraction of full power that's needed to maintain freeway cruise speeds¹.  Since the power output is proportional to fuel consumed², your car engine uses much less fuel.  Even though it might be rated to produce the same max HP as the airplane engine, in normal operation it produces much lower HP for the vast majority of its operating time.  You can see this in operation if you drive a car with a real-time fuel consumption gauge.  For instance, my Insight hybrid shows ~25 mpg when accelerating or climbing mountains (using near full power), but 75 mpg or better cruising on a level interstate.

Also note that the actual fuel economy of an airplane isn't that much different from say your average SUV.  I don't have the exact figures any more, but I once calculated that my Cherokee 180 got about 18 mpg.  When you figure that it's going twice as fast, and on longer trips can shorten the actual distance travelled by going in a straight line instead of having to follow roads, the comparison isn't that bad.

¹This, incidentially, is why hybrid cars can use electric assist to get much better mpg - the gas engine can be just big enough for efficient cruise, while acceleration is provided by the electric motor.

²To a first approximation.  It's a lot more complicated in the real world.  Look up BSFC maps if you're curious.