The NEW Cayenne is much better on gas than the old one, my 05 S sometimes 10 to 12MPG for the whole tank. the new one according to PCM, it's 18MPG but realistic driving is only 16MPG, I have filled about 4 tanks, every tank PCM says about 18, or 17.5, but actually calculation (every time I fill up the tank I reset odometer, and when I fill tank again, I use the driving range divide number of gallons filled, which is the most accurate way of calculation), it's always, 1.5mpg to 2mpg difference. which means PCM always calculate 1.5 - 2mpg more than actual driving, any guys experience this?

 

Seems most people just believe the PCM/MFD, but I too calculate mine manually as well and have found it to over report by as much as 8%.

Given that the MPG is a major selling point of the CD I was disappointed to find my first tank that said 24.6 on the MFD was actually 22.5.

I have noticed that the higher MPG I get, the closer the computer is to the truth. It always over reports though.

Another quirk I found is that the MFD and PCM (Car module) will disagree about the MPG (by 0.1) and Average Speed (1 MPH). It's not like one happens to switch just before the other either. I drove it 30 miles last week with the speed off and when that finally decided to match, then the MPG was off again.