Just as another measure for you, with my JDM model [3.2L sitting in 2High - rwd] for the tank according to my Japanese owners book, I can get 85L of fuel in and the fuel light comes on at 8L left, I refilled mine when the light came put 76L and travelled for 530KMs, mix of short round town plus some 25-50km highway runs, not revved over 4k, Power button on when i remembered..so according to my fuel calculator..that's ~ 20mpg, the premium fuel I use is 95 octane, the best available here is 98 octane ....but is like another 6 - 8 cents a litre more, taking that fuel to nearly $1.20 AUD per litre !!, so I just opt for 'regular' premium..
..also my VX even with 20s at 275/45 are supposed to only give a speedo error of 1 - 2% tops, but its actually around 5% [used GPS plus 2 other cars to compare speed to indicated, speedo shows more speed than actual road speed], ....the speedo error was even worse with the original 16s close to 10%...anyway ...based on speedo error..around 19mpg..