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Typically, peak engine efficiency happens at low RPMs and heavy throttle. Opening the throttle plate, by itself, improves efficiency, but if you press the gas pedal too hard, the ECU enriches the fuel mixture and quickly eliminates your savings. My Subaru (2.2L 5MT) experienced quite leisurely acceleration and relatively good gas mileage when I used 85% throttle and upshifted at 2000 RPM.
Tires: it's safe to go up to the sidewall max pressure. The ride is harsher and noisier, but I think you'll like 51psi anyway.
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