Originally Posted by kam327
True, but all I'm looking for is the long term trend. Shouldn't matter if they're city or highway drivers. City drivers' mileage will vary more widely on a given tank but over 40 tanks or more the short term variance will disappear and we'll see over the long term if their mileage is trending up, down or steady.
You would have to assume that they trend one way or another throughout the graph though (city vs. highway). What if, for one the drivers, he drove the first half of his fillups on the highway, and on the last half of his fillups he drove in the city? ...or some other combination. Would that be good data for your study?