There's no set RPM level when driving that offers optimum mileage. In general, the lower the RPMs a tachometer is showing at any given speed, the less fuel is being consumed. Going 60 miles per hour at 1800 RPMs will deliver good economy. Going 30 mph at 1800 in the same vehicle is guzzling gas.
Though driving with your transmission locked in a high gear will lower the tachometer reading, your fuel economy will suffer if you are lugging the engine. Lugging means the engine has to work overly hard to just to maintain speed. If you have to keep the accelerator almost to the floor just to maintain your speed, you car's tachometer reading may be low, but your gasoline consumption is high.
Most of the time a low tach reading is a sign of good gas mileage. Your driving habits and the mechanical condition of tires and engine also affect your mileage.
