Crappy MPG 991.2 C2S
#1
Track Day
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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.
#2
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Are you actually measuring consumption, or using the car's own readout? The latter has frequently been called less-than-accurate.
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Hatchbm (05-06-2024)
#3
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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)
#4
RL Community Team
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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!
#6
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Yes, I had a PDK in the C4S. I forget, are ratios much different between the manual the PDK?
#8
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The mileage is oddly low.... give it the italian tune-up a few times and see if milage improves afterwards?
#9
Pro
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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?
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?
Last edited by RennListUser01; 05-06-2024 at 06:22 AM.
#10
Nordschleife Master
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Do you have aftermarket wheels with different offsets from OEM?
#12
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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.
#13
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Not sure the fuel grade or source would make much difference?
Tire pressure?
Not running 'comfort' pressures, rather the higher 'max load' pressures.
Comparative rpm at 70?
2k-ish
Air filters (although hard to imagine that much effect).
Fairly new, but fuel trims indicate more air than expected.
Certain of the amount going in - topped off as close to exactly same each time?
Yes, filled up.
Can dealer be talked into a full scan to check O2 sensors, trim, etc?
Tire pressure?
Not running 'comfort' pressures, rather the higher 'max load' pressures.
Comparative rpm at 70?
2k-ish
Air filters (although hard to imagine that much effect).
Fairly new, but fuel trims indicate more air than expected.
Certain of the amount going in - topped off as close to exactly same each time?
Yes, filled up.
Can dealer be talked into a full scan to check O2 sensors, trim, etc?
#15
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timothymoffat (05-06-2024)