1900 RPM is about optimal. There's a mix of gearing, boost levels, and highway drag that can change this number. 2000 works too, 1800 rpm works too. It's important to not lug the engine though. Lugging the engine is puts more stress on the engine than higher rpms in a lower gear. This is why the 5th gear mod works - you can stay in an area of optimum fuel efficiency without lugging the engine. RPM goes down, load goes up, but the overall effect is positive. However, many people don't observe any signifigant fuel efficiency gain because they drive faster. That's exactly what I did - the car was quieter and rpms were lower so I just drove faster and it wiped out any difference in fuel economy!