We offer that in the mini-stats section of vehicle profile pages so you can add it there if you want. There aren't a large number of people using Gallons/100 miles to compare performance like there are with the other options. So until it becomes more of a standard that people want to use we're fine with offering it on vehicle pages for members, but not across the entire site.
I think we should start promoting the gal/100 mi measurement to become a standard. The signature badges should be a good start. This way, the true consumption of fuel can be measured easier - the volume/distance measurements will highlight fuel inefficient vehicles.