do not associate rpms directly with fuel consumption. it's far more complex than that. the basic goal is to keep your engine in its optimum torque range because that's where the best fuel economy happens. Lets assume that your current setup was ideal (it's likely already running at a bit lower rpm than ideal). when you install 33s, in order to maintain a given speed the engine will now be at a much lower rpm which is now lower than optimum and now the engine is working extra hard to maintain that same speed (meaning it's consuming more fuel). adding in lower gears (higher number) brings that back up to the optimum. keep in mind that bigger tires are, well, bigger. they weigh more and have more friction with the ground and mpgs can suffer num matter what you do. the key is to get the best bang for the buck.
tire diameter and gear ratio are directly related to each other when it comes to both engine performance and mpgs. when you change one, you need to change the other. :thumbsup: