Sorry, there's no way you did 13L/100KM. Assuming you did get 18 MPG that converts over to 15L/100KM at best. On my last fillup my 5.4 did a combined city/highway of 16.4 MPG or 17L/100KM. Your 4.6 won't do that much better.

 

I like MPG but I can't stand driving as slow as some of you guys are driving on the highway. If the speed limit is 65 I do 75. If it's 70 I do 80 and with my old tune & 4.10 gears that equatted to around 15.9mpg. lol Driving 70in a 70 just takes too long to get anywhere. I feel like driving Miss Daisy at that speed, especially on a 7-8hr trip.

 

The first tank full in my 2008 4.6L 3.55 4x4 was 20.1 mpg. I had about 1000 pounds in the bed for 100 miles on the second tank 17.6 mpg with the OD off.
Third tank was 19.5 mpg.