At first blush, I'd say of course you can. Let be the fuel efficiency, in MPG, for day Let be the volume of gas consumed, in gallons, for day Then the average fuel efficiency for week one, in MPG, can be computed by a "weighted average" as follows:

Note that is the distance traveled for day So, basically, what I'm doing is not computing any of the intermediate distances, but plugging, in turn, their formulas into the final formula. See how that works?