Whenever I get a new car I want to know exactly how accurate the fuel gauge is.

When it says 10 miles I will drive to my favorite station and drive a 4 block circuit around it, watching the gauge until it gets down to 2 or 3 miles, maybe even zero. Then I will fill it up, topping it off to the lip. Check the receipt to see how much went in and subtract that from the fuel tank capacity listed in the owners manual.

The answer is the real-world reserve when the gas gauge says zero.

 

Very interesting Sid. Thanks for posting.

Here's a question extending your info about the gas staying cooler and its benefits. Btw, sorry to the OP for digressing somewhat.

Apart from the adverse effects to engine and components, how does the addition of ethanol affect gasoline temperatures?

 

I guess the Valet took the car out for a drive and didn't put any gas..... There is no problem with the sensor... ;-)

 

Wow who knew gas temps would be this interesting... you bunch of brainiacs!

OK here she goes...
It has to do with the energy released when combined with an oxidizer and the increased efficiency parameters of the new DFI (and it's variable spray pattern/throttle mapping)... something along the lines of cooler temp gas has a higher oxygen content because the liquid gasoline remains further from it's vapor point.
This also results in a higher activity rate between the protons and neutrons because of the bigger differential between the cold(er) liquid state of the gasoline and higher temps of vaporization before ignition.

I'm paraphrasing but that's more or less it... kinda like the physics behind released energy derived from poring hot water on a block of ice vs cold water... one will release enough energy to crack the ice the other will just melt it.
Or a collection of particles where energy and momentum conservation dictate that both collections of particles have the same energy and the same momentum... not really but kind off.
But I'm sure some of you rocket scientists and engineers can do a better job of it than I just did... I sure hope so!

Clearly fuel temps have a minimal effect compared to other scenarios where temp variants have a much more pronounced effect on our engines like higher temp ambient air being fed in through the cold air intake etc.

The real issue as it pertains to running your tank dry of course is the fact that...uhmmmm....well...it's just not a very good thing to do...it can really create some significant issues with the DFI and other highly sensitive components, if not at minimum leave you with a clogged fuel filtration system which in turn will cause it's own set of issues...

Re; the gas level readings.. this is the way it used to be with 996 and 997, I'm not sure if that is still the same with the 991.
I know the basic split tank design is (the same) but like so many other idiosyncrasies it could be that it has gotten more refined over the generations culminating in a virtual non issue regarding said idiosyncrasy in the latest achievement in technological and digital advancement... the 991.

However in this case, I think M5_2010 may actually be one the money!
Schunmd did you check your odometer before and after the valet parking?
Seriously...I think that's your problem right there!

 

Yah, Ferris Bueller's Day Off!!!

It's a good thing I remember mileage before I turn the key over every time I valet my cars, thanks to that movie!

 

If it was the key, why would it even crank?