Fuel Efficiency Dilema

Hi you engineers out there:

How Efficient can a Car be in using gasoline?
What would the temperature of the engine be when all gasoline is reacted with oxygen in air in every engine cycle for an hour to produce only water and CO2?
What would be the temperature of the engine?
How much more Nitric oxides would result at such an eleveated temperature?
How much more of these gases would escape the catalytic converters into the atmosphere as nitic acid?
Can't we sacrifice the weight of the car (go for smaller, composite material-based cars) as a measure to lower fuel consumption?

J. Alila