It's good to remember that mileage isn't everything; mileage is just one number. Another, possibly more important number, depending on how you look at things, is total fuel consumption. Going 5 extra miles in conditions that favor high mileage to warm your engine up might get you a higher mpg for that trip, but with the extra five miles, you might use more fuel than a shorter trip at lower mpg.

Another thing to think about is how much of an [effective] hypermiler someone who carpools is. If I drive by myself, I get about 50 passenger-miles per gallon. If I carpool with one other person in my car, and drive the same way, then all of a sudden that's 100 passenger-miles per gallon. A car full of me and four passengers? Somewhere over 200 passenger-miles per gallon (I won't be getting 50 car-mpg so much with that many passengers).