Fuel Economy Decline in Ford Mustang 5.0

Not that I was ever expecting to get great gas milage from my Mustang, but over the first 30,000 miles I have noticed a decline in fuel economy. About 1 MPG over the last few months. I’m following the maintenance schedule and while an increase in local versus highway driving might be the culprit, in the past, when I have been driving more locally, the MPG average hovered around 15-16, now its around 14. Any thoughts or suggestions?

We cannot comment on that unless you describe in detail how you measure gas mileage and how and where you fill your tank.

It’s easy to be 1 mpg off and the reason could be one of many.

I measure gas mileage two ways: with the onboard computer display that indicates average MPG and through the Gas Cubby app on my iphone which calculates it every time I fill up. I usually buy my gas (87 octane) at Sams Club.

Do you ever continue to fill the tank after it the filler clicks off? That can cause inaccurate readings.

If you are filling up at the SAME PUMP at the same station and stop at the first click there should be no variance in the accuracy of what actually goes into the tank. Pumps at the same station can vary significantly as to when they turn off the gas flow. However, driving environment and style has the biggest influence on gas mileage.

My Toyota gets 38 mpg on the highway and only 30 mpg around town. The mileage goes down in the winter for obvious reasons.

Your actual calculation of miles driven divided by gallons should be more accurate than the on board computer.

I suspect your change in driving pattern is mostly responsible for the drop.

1 MPG is nothing. It’s statistically insignificant. It could be that you’ve become more comfortable driving the car, and you’re driving it more aggressively without realizing it. It could also be that you are overdue to check your tire pressure, or that weather patterns have shifted and you’re driving into the wind instead of with the wind, or one of 100 other possible causes.

If your fuel economy drops by 5 MPG, it will be time for some diagnostic work. This could just be a statistical anomaly.

Thanks for all the comments! I agree that it likely a combination of of-highway driving and weather conditions–we’ve had a particularly brutal summer in Austin and I’ve had to blast the AC on max most of the time I’m driving (we’ve had about 40 100+ degree days so far. I’ll keep an eye on it though and check back if it gets worse.


This is how I calculate my gas mileage

Fill up tank
When it clicks, I stop
Reset trip meter

The next time I fill up until it clicks, do this
Divide the number of miles driven by the amount of fuel needed to top of the tank
At least you know what the fuel economy was for that “trip”

Perhaps you can try it once my way. If “my” way has the same fuel economy as your onboard computer display, then at least you the display is pretty accurate

Also, the new 5.0 engine is designed to run optimally on 92 or 93 octane gas.

Where I live, I’m not even aware of stations carrying 93 octane

You seem to be doing better than most. According to the testers at Cars.com, 12 mpg is about average for this car.

how old is your mustang? 5.0 has been around for a long time.

I’ve never put 92 or 93 octane in the car…the owners manual recommends 87.


What year is your Mustang 5.0 . . . ?


I just read your other posting about the performance modifications, and noticed that your Mustang is a 2012 model year.

You never mentioned this in your “fuel economy” posting, and some of us assumed you had an older 5.0 Mustang from the 1980s or 1990s

The reason we assumed this is because the 5.0 designation was gone for several years, until Ford recently revived it.


I just read the 2012 Mustang owner’s manual online

It clearly states that 87 octane fuel is recommended. It doesn’t mention anything else.

Please post the source you have that states that 92 or 93 octane is recommended for optimal performance

I would like to read it

I know my car is older, have an 89 Mustang GT 5.0 with a 5 speed…113k on the clock…On the highway I average about 20 -22 mpg and around town about 15 mpg. The only thing I did on this car was go to 3.55 gearing in the rear end…sort of in the middle where you get a bit more power off the line but not enough to effect gas mileage from the standard 2.73 gears. I can now shift into 5th gear around 40 mph vs almost 50 without lugging the engine.

You have a 8 cylinder hungry V-8. I found out that, just drive the vehicle “normal” an no flooring it for the best mileage, shifting comfortably into the next gear. You new 5.0 has more HP than my 89 so its a bit hungry for fuel. I bought my 89 brand new for 12k and still have it…not too many can say they have an original 24 - 25 year old car. I have not changed anything under the hood except for a water pump and starter…Had to put in a new clutch at 80k due to a bad throw out bearing and not due to abuse and still have the original ignition module on the distributor… Change the oil every 6 months and runs great on 87 pump gas and never drag race my car. A lot of kids see the 5.0 on the car body and won’t waste their time . The 5.0 was one of the best engines ever made…from a stock 302 to the H.O. Windsor 5.O H.O. engine.

For the best gas mileage, you need to shift into the next gear just before the engine would start lugging.

Drive as if you had a raw egg under the gas pedal and you will see an increase in mileage. .There is no reason why you can’t drive this car without going past 1/4 throttle for normal driving.


If you look here

The 5.0L only makes it’s advertised 420 HP when running on premium. IIRC it’s around 400 when running on 87 octane.


Thank you

If it were my car, I’d put in the recommended 87 octane and be happy with “only” 400hp . . . !

After all, the owner’s manual straight out says to use 87