The key is carrying enough inventory ( I do get poor gas mileage) but once I'm out in the field I never have to stop at a supply store to pick up parts as I carry everything needed on the vans.
What good is getting 30 MPG if every time you have a call you need to pick up materials that is time and money wasted.
Here I get an average of 4 MPG but have very little miles on the work vans as I have no wasted trips.