So my car, 134 hp with 314 lb torque, acheives 7.5 litres on constant velocity highway driving at 3,000 rpm.
Both power and torque vs. rpm are not linear curves, and differ with the engine internal characteristics such as spark timing. A typical aero engine will deliver max power and torque at 2700 rpm or thereabouts, where a typical car engine will deliver max power and torque around 4-5000 rpm (diesel) or 5-6000 rpm (petrol). Motorcycle engines and F1 racers occasionally can go as high as 13K rpm, as I recall. So comparing the rpms will not work either.
What you need to look for is the SPC figure - Specific Fuel Consumption. It might be a bit hard to find or calculate, but is essentially the liter per hour per horsepower figure. And is possibly measured or given for a specific (most efficient) rpm only.
Could it be something to with the fact that your aircraft engine is constantly generating the energy to overcome drag and to generate lift. The car engine only produces lift when going up hill?
Indeed. An aircraft needs to produce lift, where a car doesn't. But furthermore wind resistance (parasite drag in aviation terms) increases with the square of your velocity. So if your car was the same basic shape as your aircraft (not counting induced drag from the wings here), then your car doing 50 knots would only have 1/4th of the wind resistance of the aircraft doing 100 knots. On the other hand a car also has roll resistance from the tires, but that will be nowhere near the induced drag of the aircraft.
I once did the calculation in a different way. I found that your typical 4-seater (O-320, 160 HP, 30 l/hr for 100 knots) will do about 6 km for each liter of petrol (assuming no wind), where a typical petrol 4-seater car would do 13 km or so for each liter of petrol. Considering that the aircraft is twice as fast and needs to generate its own lift, I think that's not too bad at all.