It is a common misconception that the number of revolutions per mile (RPM) has an effect on fuel economy. In fact, fuel economy is determined by a variety of factors, including tire size, engine displacement, and drivetrain configuration. The RPMs at which a car's engine turns have no direct bearing on how many miles per gallon the vehicle will get.
There are some indirect ways in which RPMs can affect fuel economy. For example, if a car's engine is revving at high speeds constantly, it will be working harder than if it were idling or cruising at lower speeds. This increased work output will result in higher fuel consumption. Additionally, if a car has larger tires, it will have to spin them faster to maintain the same speed as a car with smaller tires. This increased rotational speed means that the engine will be working harder and using more fuel.
Ultimately, though, the best way to improve your gas mileage is to focus on driving habits rather than worrying about RPMs. Things like accelerating slowly, coasting whenever possible, and maintaining steady speeds will all lead to better fuel efficiency regardless of tire size or engine speed.