Just thinking out loud…

Consider a 2 litre petrol engine, idling at 720 rpm or 12 revs/sec. It takes two revolutions for each cylinder to complete its cycle (4 stroke assumed), so 6 cycles/sec. The amount of air/fuel mixture drawn into the cylinder(s) is 2 litre per cycle, or 12 litre/sec. At 7200 rpm it would be 120 litre/sec. In general, the figure is (RPM * engine capacity) / 120.

The modern engine management unit computer goes to considerable lengths to keep the air/fuel mixture constant, and close to the so-called stoichiometric ratio — the ratio at which the fuel burns completely. A little research tells me that this ratio is 14.7:1 for petrol (different ratios for ethanol, propane, diesel, etc). The actual ratio is usually set slightly richer (lower) and may vary slightly, between about 12-14 or so. The ratio cannot vary much more than this without either wasting fuel, failing the emissions test, or damaging the engine.

This ratio is expressed in mass/mass terms, but we want litre/litre. The density of petrol is roughly constant at about 737 gm/litre. The density of air varies according to the pressure and temperature. At “standard temperature and pressure” it’s about 1.2 gm/litre. It’s a matter of common knowledge that the air going into the cylinder is warmer but lower pressure than outside air. The temperature usually doesn’t change much, but the pressure can vary quite a bit. At present, I have not found a reliable source for this information.

Now we have consumption in litre/sec but we want consumption in litre/km. The factor here is road speed. However, road speed is directly linked to RPM by the gear ratio and the size of the tyres. For our purposes, we can now replace RPM by “engine to road gear ratio”.

So, in summary the instantaneous fuel consumption of a petrol engine in a car is governed largely by just 3 factors: engine capacity, engine to road gear ratio and inlet air density (mainly pressure). So, I did some quick calculations and I do indeed get reasonable figures, but only if I assume an air density that is quite low.

On the subject of pressure

In normal driving, the inlet pressure is low when the throttle is closed (foot off accelerator), starving the engine of air. It is higher when pulling under power, such as going up a hill.

The important pressure to consider is inside the cylinder at the end of the inlet cycle, just before the inlet valve(s) close. To get more power raise the density by raising the pressure. Larger or double inlet valves will help, but this is the main function of a turbocharger or supercharger: to raise the pressure.

Conclusion

To improve fuel economy at a given speed choose: smaller engine, lower gearing, lower inlet pressure. Nothing else matters (much). Other things being equal, an engine twice the size will use twice the fuel at a given speed unless the RPM is halved.

One final point

One factor I overlooked is the amount of residual exhaust gases at the end of the cycle. They act to effectively reduce the cylinder capacity by 10% or so.