Steve - it looks like the db does both. If you click on a particular vehicle type you see a listing of all members and the "lifetime" average. If you then click on a particular member you can see the "tank-by-tank" averages.

For example, in the Highlander Hybrid 4wd section "Bluefin" shows an average mpg of 27.0. If you click on "Bluefin" you get to see his/hers five tank averages 26.3, 27.5, 28.7, 24.5 and 28.2.