I highly doubt you will see that mpg. My dads challenger with the 5.7 and cylinder deactivation only sees like 25-26mpg on a good day all highway. 4.10's will also bring up rpms on the highway so that will make you loose highway mpg

 

Id say if you get 20 you will be doing good. They just have started seeing those numbers from the newer engines with cylinder deactivation so I wouldn't expect those kind of numbers. Now if you dropped in a 4bt then oh yeah