dprantl

My GT gets 21-22mpg on 100% highway cruising at part-throttle 75mph. It is supercharged but since the S/C is bypassed at vacuum, it is probably affecting economy very little. I would find it hard to believe that the auto transmission is slushing away 4-5mpg... I would first check the O2 sensor since it is responsible for keeping the mixture at 14.7:1 when cruising.

Dan
'91 928GT S/C 475hp/460lb.ft

G8RB8

Last time I checked I got 17.2 on a 400 mile turnpike cruise. I run around 80 and occasionally sprint to 100+ for a couple of miles.

MFranke

All of mine so far seem to get around 20mpg plus or minus a few digits depending on how I drive. My commute is about 1/3 city or residential and 2/3 highway. Most of the time I'm just cruising but with some fun thrown in here and there.
Long road trips seem about the same gas mileage - maybe because I'm usually going faster it offsets the savings.
I haven't seen noticeable difference between the autos and the GT yet in gas mileage. The GT has less drivetrain loss but then again operates at higher rpm for given speed. Maybe that offsets things. They are all in good tune.

KenRudd

I get 12 to 13.
I drive it fairly tamely on the street.
On my list of things to investigate, not because I care about the actual MPG, but just to make sure it is not symptom of something else not in tune.
Might just be old car+ Porken chips.

Bill Ball

Over all is 16-18 MPG. Highway 20-21. Always has been in the same range.

eijun

It is way easier to go down in mpg than go up, but it is easy to keep it within the same mpg category with just day to day preventative maintenance.

I see it like this:
22 & above (likely your car does not have a spec of dust in and out)
20-22ish - Above Avg (good long roads & finely tuned for mpg, and likely good condition)
17-20ish - Average
15-17ish - Little below Average (usually driving habits or road types)
12-14ish - Car mods for performance that don't care about MPG, but it could also mean you need an overhaul of the car being it is in bad shape and needs parts (at least look into the vital parts that affect your mpg)
11 & below - Get your butt to the damn mechanic and spend 10k on parts and a refresh.

Leon Speed

It is there any way to check the O2 sensor? What other components determine A/F ratio?

Lizard928

The O2 is a wear item, replace with a direct plug in Bosch unit.

If the car has run rich for awhile, this will kill the O2 sensor.

Dinsdale Piranha

Our MY82 averages 23.5 @ 80mph .

jpitman2

I get around 17 on a cruise in MY 83 CIS auto, and 2.75 rear end.
jp 83 Euro S AT 55k

sweet928

OB's get 10mpg. Says so right on the window sticker too. CIS was quite inefficient I guess.

Schocki

My GTS gets easily 20 MPG on the highway, best so far was 24 MPG at 75 MPH.
But my GTS is equipped with an US S4 ring & pinion....

Rob Edwards

Actually, the one legit datapoint I have on the stroker was 17.9 mpg over 1000 miles to and from SITP 2011.