the optimum ratio will be the smallest amount of fuel injected to make enough power to keep the vehicle at cruise, just slightly above the start of your torque curve.

 

That normally translates to 17-1800 rpm at whatever your desired cruise speed is for an unloaded truck in top gear.

 

not necessarily. the engineers are trying to make an engine that can work just about anywhere for anyone where the vehicle is sold. elevation, temperature, fuel quantity, fuel pressure, cylinder pressure, and probably most of all - emissions, all play part in determining what timing to use.

MPGs are obtained by advancing timing "just enough" and pulling fuel to just the amount needed to maintain speed (this is normally done by the engine/pedal already, otherwise you'll pick up speed while cruising) and adjusting fuel pressure to modify burn speed. you want a burn speed that pushes on the piston in the right direction for the most number of degrees without wasting too much energy out the exhaust.

the trick is to find the right balance between: timing, fuel pressure and fuel quantity. work the first two and the quantity should come automatically.

higher fuel pressure makes the engine run like it has more advance timing. it gets the fuel in faster and atomizes it better so it burns quicker. lower pressure requires more timing and vice versa. somewhere in between is your optimal burn for mpgs