My own experience contradicts a bit...
'01, 5 speed, 2.0, 2 door, stock, 116k miles: I found I got about 2mpg better with the stock 205/75/15 tiger paws than the 215/75/15 bfg at's that I have now. I think part of it is that the tires are more aggressive and heavier. My speedo was also dead on with the 205's at 70mph and is now reading slow by about 1.5mph at 70 compared with my gps. With the 205's my record was 34, but I would regularly get 28-30. With the 215's my record is 30, and regularly get 26-28. This is all with conservative driving and my mileage drops about 2mpg in the winter. I don't think i've ever had a tank below 20mpg even when some four wheeling was mixed in!