08 Cayenne owner, computer / gas mileage issues Hello. I just bought a used CPO 2008 Cayenne S with 35,000 miles on it.
It seems to run fine and I have about 1500 miles on it since I bought it.
I have two questions.

#1. My mileage at 75-80 mph over 700 miles clocked in at only about 16.75 mpg. An even mix of highway and in town driving only gives me about 15 mpg. How does this stack up with other 2008-2009 Cayenne S owners? BTW, I drive with a light foot and haven't used the sport mode.

#2. The on board gas mileage computer has proven to be off 1.6 - 1.8 mpg in it's calculation (giving a deceivingly good mpg report) compared to the actual mileage calculated by using the dash odometer and dividing the number of gallons of gas used at fill up into the mileage. The dash odometer and odometer on the Navigation screen agree to within less than 1% error. I have winter tires on it but they are within .1" of the same height as the OEM tires. I know it effects horsepower but does fuel octane effect MPG?

Any thoughts or input is appreciated.

 

That sounds about right - I would say over the long term/mixed driving I see averages around 15mpg on my '04S ... on 100% highway trips with the cruise set and little or no slowing, high teens. Don't think I've ever actually seen 20mpg.

With that said, the newer models are supposed to (maybe) eek out a bit more mpg's, but I can't see you seeing much different than me, given that we're driving a big 'ol brick with a nice big V-8!

 

Have you compare the computer gas mileage computation with a manual one of your own?

 

80 mph is light on the foot?

 

Ha! I meant that I'm rarely under WOT. I try not to waste gas with unnecessary acceleration. Driving I-80 for 10.5 hours at 80mph saves close to an hour compared to 75mph (speed limit) and the cops will almost always leave you alone.

 

I too have found that the computer mileage is 10-15% more optimistic...the miles traveled is dead on but the manual mileage and computer mileage comparison is pretty far off....and not in the direction I'd like!

 

I've asked the Porsche dealership if they what would cause the discrepancy and they did not know. I agree, both odometers measure very close, so where is the error occurring? From fuel used from idling possibly? I don't think I idled hardly at all in a recent ten hour interstate journey.