I've only got about 430 miles out of my 3.0i in the condition that you've described (driving from LA to eastern Sacramento) before the fuel light came on (I'm assuming that the 10% incline is the climb over the grapevine out of LA?). But I did push it north of 85-90mph at some points during the trip. The trip computer say I only average 20.0mpg during that entire trip...meaning that I should still have about 2.5 gallons left in the tank when the light came on. You should be able to squeeze a bit more out of a gallon of gas (as much as 6-7 miles per gallon more) if you stay under 70mph as I found out over the weekend.