gas mileage vs. throttle position

[neosec]

In short... Is the throttle (gas pedal) position directly and proportionally related to gas mileage in newer computer controlled cars?

I ask because...
I have a 1998 Jeep Cherokee, lifted with 31 inch tires. The transmission has not adapted to the larger tires by downshifting on (up) hills, so I downshift manually.

Example: I'm going up a hill, the tranny is in OD (4th), speed 45, RPMs about 1500. Now, without moving the gas pedal I downshift to D (3rd) and the RPM's go up and the speed starts to climb. Am I using the exact same amount of gas as before the downshift but going faster thereby getting better gas mileage. Or does the motor use more gas at higher RPMs regardless of throttle position?

Neo

[TheSilentChamber]

Higher rpms = more gas used.

[beef_bourito]

it depends, there is a certain rpm at which your engine is most efficient. you'd need certain graphs and knowledge to understand them before you could tell. if downshifting put you at this point then you would be using less fuel than normal. a general rule of thumb however is the lower the rpm, the lower the fuel consumption and visa versa.

[curtis73]

The amount of fuel your vehicle ingests is based on a few things. Pulse width of the injectors (how long they're open injecting fuel), throttle position (since the more air you let in, the computer compensates with more fuel) and RPMs (since the faster you're going, the more times per minute those injectors fire letting fuel through.)

In general, the more rpms, and the more throttle, the more fuel she's guzzling. It logically stands to reason that the most fuel gets used when you have it at WOT right around redline before it shifts up.

Like beef said, there is an RPM where your engine makes peak efficiency, so at that RPM the engine would take less "foot" to maintain speed, and therefore MPGs are often maximized at that point. Anything below that peak and the engine isn't making as much power, so it takes more foot to maintain speed. Anything above that peak is just spinning faster (and injecting fuel more times per minute) than you really need.

Find where Jeep advertised peak torque for that engine. If you cruise at or about that RPM you should be close to best MPG.

Edit: to more concisely answer your question, ya sorta need to know some specifics of what is happening in the engine. By shifting up you say you needed less foot to maintain speed so its injecting less fuel per pulse, but pulsing more often. I guess it depends on how much of the pedal you let up versus how many more RPMs you gain.