NeinElf

My 2017 911 991.2 C2S (3.0L + 7MT + RWD) is returning pretty mediocre fuel mileage on the highway, even when running with Sport mode off and cruising in 7th gear. If I'm steady at 75mph, economy seems to sit around 22mpg on Chevron 91. In contrast, my old 997.2 Turbo S (3.8L + PDK + AWD) would return 23-24mpg when when averaging quite a bit faster. Looking at the fuel trims when data logging on the 991.2, long term fuel trims seem to hang out between 14% and 17% (positive) on both banks and I'm not sure what I should look at next to troubleshoot. There has been no check engine light or other issues, car seems to make good power. I see other people on here talking about high 20s mpg, which I could only achieve with a 50mph tailwind or a long downhill run.

Larson E. Rapp

Are you actually measuring consumption, or using the car's own readout? The latter has frequently been called less-than-accurate.

NeinElf

I've done the math manually. Normally, I can only squeeze 240 miles or so out of a tank in mixed driving, so it's definitely not very good on gas. (averages 14mpg in town)

asellus

How are you calculating mileage? The MFD's accuracy is questionable. ope, posts happened
Did you buy new or used? If new, did you tune it?
Is this a new behavior?
Has the exhaust been modified? Upstream o2 sensors come to mind, but having both conk out like that would be odd.

I used to get 24-27 on the freeway in my 2017 C4S, using Costco 92 AKI fuel. The APR stage 1 tune made no change to my mileage. I calculate mileage using odometer + fuel pumped, not the MFD's readout. I have had exactly one car where the displayed MPGs were accurate, and that's only because it's open to be adjusted!

NeinElf

Purchased it used as a CPO, it's stock all around, no tune, no MIL.
Was the C4S a PDK?

asellus

Yes, I had a PDK in the C4S. I forget, are ratios much different between the manual the PDK?

arter

hmm on a long highway trip I can get 28+ mpg at 78 mph with my 991.2.
Range well over 400 miles.

spdracerut

The mileage is oddly low.... give it the italian tune-up a few times and see if milage improves afterwards?

RennListUser01

Not sure the fuel grade or source would make much difference?

Tire pressure?

Comparative rpm at 70?

Air filters (although hard to imagine that much effect).

Certain of the amount going in - topped off as close to exactly same each time?

Can dealer be talked into a full scan to check O2 sensors, trim, etc?

Porsche_nuts

Do you have aftermarket wheels with different offsets from OEM?

rensoyka

a little over 400 miles per tankful all highway on cruise control, all city sucks at around 250 miles in sport mode.

vanlieremead

30 on the highway, 22 around the rest of the time. I presume you are up on all maintenance but mine has always been at that level regardless of before or after plug changes. Get your self an oil sample at Blackstone on the next change.

NeinElf

Fuel trims are weird (adding 15% more fuel), I'll probably have to take it in to look for vacuum leaks.

NeinElf

No, OEM Porsche 911 991 RS Spyder 20" with PS4Ss

NeinElf

It gets run very hard on back roads, sees full loads & redline often. It's fully Italian tuned.