got a 2014 mazda 3. my question is the 'current mpg' on the trip computers. it changes every 2 seconds and goes from a low reading to as high as 99.9mph. with so many readings how do i know what is accurate? how is the current mpg calculated and why so many up and down readings?

Because it is the "current," or instantaneous, MPG. It is exactly what it says it is. Every time you give it a little more, or a little less gas, your current MPG changes. Are you getting that confused with your "Average MPG?"

What you're seeing is just one of four functions of the trip computer. You switch between current fuel mpg, average mpg, estimated distance on remaining fuel, and average speed by pressing the INFO button on the steering wheel.

Yeah, what the previous two people said. Those were both excellnt answers. If you're still confused, read the owners manual that came with your Mazda.