Tyre size has a lot to do with it to.
OE 265s will return the best figures go up a size expect an increase of 1 to 2 ltr per 100 K's.
Also what tread pattern as well A HWY stlye tread like a cooper ATR will be better than a full on Mud pattern style tyre
This is for 16" rims

@ ET having the chip on 8 have you noticed black smoke more than usual,
Think mine is on 7 just for that reason