Sir, one way to measure it is to fill up your tank and reset the trip odometer. When you next fill up your tank, record the miles driven on the trip odometer. This is your miles. Then, record the gallons required to fill up again. It is not perfectly accurate because sometimes the Jeep might not fill up properly, but it should give you a good estimate. The gallons used is the bottom of the equation. So you will have Miles/gallon. Take the miles driven and divide it by the gallons taken to fill it up again. This is MPG, or miles per gallon.
Imagine if you had a clear gas tank, which you don't have. When you fill up, you could mark the line where your gas reached in the tank. When you next filled up, you would know exactly how much fuel it takes to get back to that same exact line, because the pump would display it as you fill up. This way you could tell the exact amount of gas was required and used to get back to that line. Then you could use the miles driven on that much gas, hence "Miles per gallon" or Miles/gallon. But, you don't have a clear gas tank to exactly measure it, so it is not going to be that accurate. MPG really has nothing to do with rpm. RPM is revolutions per minute. RPM is more related to speed. Where speed is linear speed and RPM is rotational speed or revolutional speed. I hope this helps, sir.
