Sorry, should have been more clear on that. Volume/distance is more intuitive for efficiency calculations. If you have an SUV that gets 12 mpg and a sedan that gets 32 mpg, and I give you the option of trading the SUV for one that gets 15 mpg or trading the sedan for one that gets 40 mpg, you'd probably be more inclined to trade the sedan. But they're both being driven, say, 15,000 miles a year, trading the sedan only saves you 83 gallons, whereas trading the SUV saves you 250 gallons.
On the other hand, if I ask you if you would rather trade 8.3 gal/100mi for 6.7 gal/100mi or trade 3.1 gal/100mi for 2.5 gal/100mi, you can immediately see that trading the SUV will save you 1.6 gallons of gas for every 100 miles you drive, as opposed to only saving .6 gallons.