Thats only true if you have an old carbureted car that requires turning a screw under the hood to adjust the air-fuel mixture. Modern cars measure the amount of O2 in the exhaust and adjust the mixture to keep the moles-of-O2 to moles-of-gasoline ratio roughly fixed.

The thinner air means you have to open the throttle wider to achieve the same manifold pressure, which in turn means that with the throttle wide-open, the engine will produce less power. But the thinner air should have no effect on the amount of gas you need to burn to achieve a particular amount of power.