MPG figures keep changing from 12.5 to 99.9

You are looking an "instant mpg" not "average mpg". When you let off the gas, your mpg could go to the maximum the display can show in three digits. That's because when coasting, you are making miles while the engine is sleeping.

You can change the display to other modes such as average MPG or trip MPG.. Read your owners manual!

Can someone actually answer. This is not while driving it does it sitting still too. It will go from 99.9 to 24,1 to 33,8 to 25,6 it is a constant change touching the gas and sitting still