spdracerut

Hm. Yeah, possible vacuum leak or maybe an exhaust leak throwing off the O2 sensor readings leading to the fuel trim adjustment.

RennListUser01

Hmmm - I like the vacuum leak hypothesis - easy to miss, easy to have cracked or pulled hose from maintenance.

NeinElf

Ratios are the same between 991.2 7MT and PDK.

NeinElf

Got my OBDII dongle working with a better app & I'm still not 100% sure what's up. I need to buy a smoke tester and see if I can find a vacuum leak somewhere.

Tpup

I just did a run. 17 C4S pdk. Avg 82 mph. Over 27 mpg.

no tune. Performance air filter. Costco premium gas.

NeinElf

That's amazing, I'd be lucky to get 18mpg or 19mpg at that speed, air conditioning off.

Jim986

I just drove two hours round trip to Hendersonville, NC and back, drove pretty slow with wife in the car and got 29 MPG.

Tpup

That's crazy imo. I'd consider finding a good Porsche Indy unless you really like doing it yourself.

asellus

I don't buy the vacuum leak.

1) these are turbocharged engines, so that means less than the expected amount of air to fuel for, not more, so fuel would be getting pulled under power, not added.
2) the 991.2 doesn't run a metered air system, it uses a MAP sensor

exhaust leak seems plausible to throw off the o2 readings, but to have it be roughly equal on both sides is certainly odd unless exhaust work was done close to the head. it would also make the car run like garbage if it was dumping that much fuel unnecessarily -- which would be the case if the readings were false.

if this is tank over tank, that generally rules out bad fuel.

could be fuel pump or filter related. check your fuel pressures? I'd expect the car to pitch a fit if pressures weren't within spec though. we're getting on in age, are the HPFPs on these up to snuff?

rnl

That's weird. I average 24.5 overall in a 991.1 and as high as 33 mpg on long highway runs at 70 mph...are you perhaps hauling gold bars? Weight can cut down on mileage. I usually store the bars before driving.

DR911s

My understanding is that the ratios are the same. I am the owner of a 2017 C2S PDK.

I usually see at least 25 MPG even when driving "sportily" in Sport mode with occasional use of Sport Plus mode. MPG does drop noticeably when the outside ambient air temperature and sunlight are such that the AC unit is working nearly continuously. But when cruising the interstate during cooler weather [no AC required] I regularly see 30 MPG or even more. I'm using Top Tier rated 93 octane gasoline. MPG is better if/when I can fill with ethanol free gasoline. I keep a complete log book for my car including every fuel purchase. The dash computer display for MPG is a little optimistic, but not more than a few percent, at least in my car. I can easily go more than 350 miles per tank, and have gone more than 400.

And I'm using comfort inflation pressures in my tires; higher pressures should produce even greater MPG. Tires have been Michelin PS4s in stock sizes. Tune improved MPG slightly when NOT using the additional power... LOL.