Jfrahm

Hi all, has anyone ever worked out how to get the best MPG you can in a 955 or 957 in an emergency? Like if a planned fuel stop is unavailable and you have to drive a fair distance with little remaining fuel.
Usually this means driving slower, I wonder if anyone has checked mpg at 40/50/60 mph or the like?

I searched and mostly found a lot of "I don't care about MPG" posts and general MPG posts with better numbers than the 16-17 mpg (USA) I get in my 957.
Cheers, -Joel.

Crozzer

There is theoretically a magic MPH at straight and level, somewhere around 40-50 MPH generally is what I’ve heard.

at idle we burn min fuel but make zero progress.
at 100 we are making progress but burning fuel fighting air resistance increasing exponentially.

somewhere in there is a sweet spot.

have you the time to do those tests on an open, flat, repeatable route and share what you find? Or is this because you are stranded with low fuel right now?

run no a/c obviously…. That’s about 5-10% IIRC, and accelerate gently, hold speed steady, etc.

phatz

Dependant upon emergency

Road Rash 1st degree
14 MPG

Viagra Priapism
8 MPG

Need a poo
12 MPG

Jfrahm

Yes, you can't creep along or you do not benefit from the overhead of running the engine, etc. So probably over 35mph.
Don't need the info right now. I plan to do some testing but I do not have a lot of flat road, and wind is a big factor here particularly where it's kinda flat. Just want the info in case I get into trouble and also out of curiosity.
I do use the Cayenne to go far (kinda fast) and then off into some remote areas so it's good info to have. If there is gas around that's not too sketch it's far better IMO to buy some than to have range anxiety.
Also always good to have enough gas to be stuck or parked for many hours and be able to run the engine part of the time.

Crozzer

yeah see the mythbusters episodes around MPG. It is quite difficult to measure precisely over reasonable and repeatable stretches of drive. I think they resorted to using a separate fuel tank and measuring before/after mass, or something like that.

you can get close by driving 5-10 minutes in both directions and being careful about using the trip computer MPG. ballpark close. set cruise at XX MPH, reset MPG thingy, wait 5-10 minutes on straight, level, traffic free road, write down the avg. repeat in other direction with same MPH. up by 510 MPH, rinse, repeat, etc.

good luck! If you don't want to be too precise, then 40 is about right.

oldskewel

I don't have first hand experience, but saw this in a movie once ...

hatchetf15

Jfrahm - For the highways: Lower Pepper all the way. No boost. PSM on. Short shift at 2300 rpm. Try 55 mph, then 70 mph. See where your instant mpg readout is best for rpm. Should be somewhere around 2300 rpm or less. I got 25 mpg at 70-75 mph with 2 bikes up top. But normal-ish Econ cruise for me has best average of 23 highway with 21 for a full freeway run.
Needed to clarify what numbers I could average, not just max.
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Add: My TTS has aero front, so not sure what that does for me.

19psi

My dad was obsessed with gas mileage. He installed a vacuum gauge that had a red/yellow/green graph in any car he owned. While teaching me how to drive, it was like a game trying to keep that damn needle in the green. It worked if you were lame enough stick with it. lol
My 928 has instant MPG which is probably the same as that vacuum gauge. Also, my last Ford truck had a bar graph that showed instant mileage. Unfortunately the Cayenne only shows an average calculated over thousands of miles, but maybe the later models have instant too?

Besides pressing the HVAC ECON button, I'd turn off any unnecessary electrical loads. Think of an old car with slightly worn individual v-belts...turn on the headlights or operate a power window and you'd hear the alternator belt squeal from the added load imposed from the alternator.

I remember reading a book or magazine article on how to conserve gas in an extreme emergency. It was something along the lines of slowly (but not too slowly) accelerating to 40 mph, put the car in neutral and coast down to 15 mph, put back in drive and repeat.

TRINITONY

Get ..

Jfrahm

I did some testing using the trip computer. It takes a lot of miles for it to settle down. In my case it's also optimistic but I hope it's proportional.
Interesting results. This is for my 2009 957 GTS with narrower than stock tires (255's on 18" wheels), MPH verified with GPS (speedo reads about 1mph high at 75).
40 mph - not good, almost as bad as 80 mph. I needed to use tip and force it into 6th or it was even worse. This is good news as going 40 sucks.
60-65 - Probably the best, but I have not been able to test 55 for long enough. Doubtful 55 is much better but I will try to find out.
75 - Not too bad, maybe 2.5% worse that 65
80 - Not good, probably 5%-6% worse than 65. Not the economic disaster I expected but the GTS likes to go 100 and I didn't test that.

Overall I was surprised how bad 40 was. Getting into 6th and running at under 1500 RPM seemed like it might be a good option for MPG but nope. The engine must be well out of an efficient RPM.
The trip computer, when reset, drops low and then the MPG slowly climbs up and dithers around the best MPG it's gonna show. Probably 8-10 miles to get it normalized.

Cheers, -Joel.

theprf

I am *shocked* by this! The best MPG I have ever managed is 18.5 with the cruise set to 72 mph (955 Turbo S). I have my air suspension set one notch low, it auto-lowers another notch I think so that is as low as it gets.

Mike Murphy

This has traditionally been the best speed range: 40-50, but it depends on the car’s drag coefficient. Top gear always. And going up hill slower than downhill. Try to not use any brakes. If a red light is approaching, coast to the light.

19psi

I'm surprised the mileage went up by forcing it into 6th. I have a motorcycle that I use for a lot of long distance traveling and hits 50 mpg at 80 to 85 mph. If I get into a slower area at 55 to 60 mph my mileage drops to 46 mpg. Dropping it to 5th or 4th brings the mileage back up. 6th gear can exceed 200 mph, so the bike is lugging the engine at 55 mph.
6th in the Cayenne is pretty tall too and I'd think it would have a similar effect at low speeds, especially on the slightest incline.

Petza914

I've never thought about this, but it's interesting. I added 100 HP to my turbo S and made it a foot wider. I have a AAA card for gas delivery if things all go wrong.

I don't think using the cruise control will net you the best result though - it will push the pedal further than what's optimal for minimal fuel usage, asespecially on an incline, it's primary objective is to maintain a speed, not minimize consumption.

Mike Murphy

An old article may give some insight