Nuke

Fuel mileage is highly dependent on too many variable to list; driver-related, car related, road conditions related, environmental conditions related, fuel related... Shall I go on?

From day one, hand calculate your fuel mileage so you know what YOUR car under YOUR driving habits along with all the other variables AVERAGE. When you see a trend (typically 5-10 data points that are inconsistent with YOUR average) that you can't account for (i.e., a change in driving habits, driving conditions, car health, etc) then begin researching.

tbear853

I have trouble seeing 20 mpg in a stock 4.6 Mustang 5 speed .... even at 70-80 .... as "good" mileage. Is that on CC or driver's foot?

At 55-60 on the highway, '08 GT coupe, 5 speed, 3.31 rear gear, mostly stock .... I can knock out 30 miles to every gallon on the road .... or better.
I don't do a lot of 70-80 mph driving as wife and I tend to use the old roads (leaving the interstate for the mindless masses rushing through life .... a habit that I acquired as a result of working the interstate for a whole career and seeing the masses out there up close and personal like .... the majority of whom are not driving, they are just along for the ride .... but I digress), but we have done some short drives like 60 miles each way at 70 +/- out I64 west etc .... mileage dropped to 27-28 range.

I can drive my '07 FX4 Scab pickup with it's 5.4 and 3.55s to Georgia and back and running 65 still knock down 20 mpg.

V8StickConv

Since my mileage seems below average, maybe I'll start with mid grade and cans of Sea Foam; if that doesn't help: spark plugs, if that in turn doesn't help o2 sensors or possibly a reprogram. Performance remains strong, only gas mileage is disappointing.

Dino Dino Bambino

I just did a 500-mile road trip over the weekend and averaged 22.6mpg (27.1mpg Imperial). About 70% highway driving between 75-85mph plus 30% driving at variable speeds cross-country including a 3000ft mountain climb/descent.

V8StickConv

I'm running a 16 ounce can of STP Motor Treatment at 1.33 ounces per gallon of gas. City mileage seems to be increasing dramatically. With this tank I'm getting more like 20 mpg city/highway (about half of each), rather than the 13mpg I got with my 1st tank. I wonder if the pintles in the fuel injectors were sticking or perhaps it's just quality detergent gas and the detergent in the STP opening the spray nozzles. The battery is 3 years old, so I don't think it could be the computer moving from default to actual driving conditions. It might also be the computer adapting from bad gas to quality gas.

Dino Dino Bambino

That's the most likely reason why your gas mileage improved. If the pintles are sticking because they've become gummed up, the fuel won't be properly atomized and consumption will increase. Quite a common issue in EFI cars that are run on cheap, poor-quality gas.
It's probably a good idea as preventive maintenance to run a bottle of fuel system cleaner in the tank every time you change the engine oil/filter.

V8StickConv

I agree. I intend to run he STP 3 tanks in a row to go for max clean out of the fuel injectors. The STP product looks to be the same as Seafoam, probably because the Seafoam patent expired, although from my reading the Seafoam uses isopropyl alcohol for a fuel stabilizer, while the STP does not. Apparently, the alcohol holds water until it's burned, while the STP product breaks the water down. I don't think this makes much difference in a car that is used regularly, but might make a difference if the car sat for periods of time, depending on where the water is in the fuel system. If anyone else wanted to comment on this, I'd be interested to learn more.

JimsGT

'06 GT auto, avg fuel economy on dash is about 21, 87 gas, 100% stock ( no tune, etc)

If I really drive conservatively, I can squeeze about 23 maybe. i used to daily drive it, 70 miles / day, all highway.

Actually Im not worried about mpg in the Mustang, unless it should ever be abnormally low.

BTW when Im driving in my Jag, you better pull over for me! LOL!

TheDutchTexan

My bone stock stang hovers around the 18 mark. But that is because I tend to put my foot in it when I can. But, with that being said, I got a longer commute, and even with the antics the computer is stating 22mpg. I found the computer to be pretty accurate, but I'll post the calculated number whenever I fill it up next.

TheDutchTexan

As a testament to the above statement:
21.6mpg calculated - 21.5mpg per computer
13.192 gallons filled - 13.2 gallons consumed per computer

42mph average speed... yes... I do drive entire stretches at 80mph, there where a couple of 80-120 mph spurts in there, had some fun with an ecoboost mustang etc. Highway mileage is where it's at. I bet if I'd keep my foot out of it for this tank I could get 24-25. (But I am not going to... I always get annoyed at myself for driving slow...)