It is a lot easier to just leave it the way it is. It is also a better indicator of "what an average Prius driver will average" if you leave those low contributors in the mix.
Now, if you want " the average of the average mpg of very committed Prius drivers" then change it.
What you might consider is getting an actual Prius mpg. All the numbers are there-distance traveled and gallons used.This could establish "bragging rights" Prius vs HCH. It would reward the mpg monsters, and the committed(ha,ha) members.Now if this number is already there,sorry I missed it.
I used the Prius as an example, of course(on my 1st cup-need at least one more before becoming hybridly correct).Thanks,Charlie