About 2000 rpm and light load is best for steady cruising. This puts the engine at about peak efficiency. Of course, if you're coasting downhill in gear with your foot off the pedal, the engine is getting no fuel if you're in full coast. If you're going uphill and higher rpm, you're using more fuel.
That's right, the car is running at best economy at about 2000 rpm and light load. You can also make sure to increase following distances so that you don't have to tap the brakes constantly. When you're driving at steady speed, tapping the brakes just turn diesel into heat and brake dust.
The issue is that if you go faster at highway speeds, wind resistance really builds up. As a rough rule of thumb, you need 4x the power to overcome 2x the drag. Drag isn't a signifigant factor until you're at faster speeds.
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!
The optimum speed depends on what car you are driving, but I am guessing the slowest you can go in 5th gear without lugging the engine. This would be about 50 mph on a level road. As others have said, you can get 55+ mpg if you drive slow, accelerate slow, and just use enough energy to get to where you can start to slow down.
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