Poor gas mileage question

[pb29590]

Good evening,
I recently purchased a used 2017 mustang GT from a dealership. It has about 1500 miles on it. The modifications that it has are a Roush cold air intake, Roush axle back mufflers and a Roush H pipe. I just had it tuned and it is making 426 HP. It was making 378 before the tune. It is running smoothly but the gas mileage is deplorable. It is registering about 16 MPG overall. The SCT tuner that was used to load the tune did not throws any codes and the gas mileage was the same PRIOR to the tune. My last S550 got much better mileage and I relaize that this mileage is grossly abnormal. Any feedback is appreciated.

Thanks

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[Anthony 05 GT]

If you are running it fairly hard that's about right. If you are driving normal with an occasional high rev or 2 it should show about 20 mpg or a little more. The MPG display is usually about 1 MPG off meaning you are actually getting worse MPG than the display shows. That's common.

The question is, how hard are you beating on it? Not that that's a bad thing. Maybe try doing your own math to calculate the actual mileage. That seems like quite an increase in power considering the mods you have.

 

[NightmareMoon]

I’ve NEVER averaged more than about 15mpg in the city, and since my commute is now only about 2 miles I’m only getting 12.x mpg on average these days. My car isnt broken or anything, I’ve gotten 25 mpg avg on long trips, but with no-highway short commute where the car barely even gets warmed up, yeah the mpg is very low.

It just depends a lot on your driving style and how much highway miles vs low speed stop and go surface streets. If you’re getting 16 I’d say thats actually pretty good for a GT. If you wanted fuel economy, shoulda gotten an ecoboost.

 

[Kong76]

I’ve NEVER averaged more than about 15mpg in the city, and since my commute is now only about 2 miles I’m only getting 12.x mpg on average these days. My car isnt broken or anything, I’ve gotten 25 mpg avg on long trips, but with no-highway short commute where the car barely even gets warmed up, yeah the mpg is very low.

It just depends a lot on your driving style and how much highway miles vs low speed stop and go surface streets. If you’re getting 16 I’d say thats actually pretty good for a GT. If you wanted fuel economy, shoulda gotten an ecoboost.

Exact same results for me as well. 80/20 city,highway hand calculated average 13-14 mpg during winter and maybe 15-16 during summer. Freeways long trips under 75 mph usually around 23-24 mpg.

 

[pb29590]

I had a 15 GT that got much better gas mileage and that is why I am posting here. I am getting about 16 MPG at 50 MPH in 6th gear with VERY conservative driving and this is not normal by any stretch of the imagination. I am searching for answers as to why this is. This is not normal. There is something wrong. I am attempting to ascertain why this is and what could be causing this. This is why i am posting here. I.have read that the oxygen sensors may be bad but it is not throwing any codes. I have also read that the intake manifold my be “cracked” Thanks again in advance.

 

[Spork3245]

What gears are on your 17 and what was on your previous?

 

[pb29590]

3.73 and 3,73. It is this the same (performance package) car- just two years later. Like I said, 16 MPG at 50 MPH in 6th gear. That is cruising speed. This is not normal behavior for a mustang. When I refill the tank and reset the gas mileage it said 180 miles to go. There is something wrong- I’m just trying to figure out what it is.

 

[Intrepid175]

16 mpg doesn't sound out of the ordinary for your car, especially with mods on it. My understanding is that the MTE readout after a fillup is based on the average mpg reading you got on the previous tank so if the car was showing 16 mpg, the MTE should be closer to ~250 MTE after the fillup. I'm not sure what to tell you about that. Is this a first fillup for you? The reason I ask is, I picked up a 2016 GT used and it wasn't showing a great average mpg reading either. When I filled it up the first time, the MTE readout was frightening but as I drove the car, getting noticeably better fuel economy than before, the MTE readout updated itself to numbers a lot easier to live with. I'd say just drive it for a while, double check the gas mileage with your own calculations and see how it turns out. If you're doing any freeway driving at all, without hotrodding it too much, you should be able to achieve something north of 20 mpg, but the biggest factor in your fuel economy will be you!

Good luck with it and Drive Safe,
Steve R.

 

[Zelek]

I average about 17-18 mpg normally with about 70/30 city/highway driving. Capable of 20+ mpg on a long highway cruise. That's with bolt ons and a tune..

I bet your car is running really rich is why. Could be the intake or MAF not dialed in right. Tune could fix all of it from a big name tuner. Who did your tune? I would data log and have them review it.

 

[Cobra Jet]

3.73 and 3,73. It is this the same (performance package) car- just two years later. Like I said, 16 MPG at 50 MPH in 6th gear. That is cruising speed. This is not normal behavior for a mustang. When I refill the tank and reset the gas mileage it said 180 miles to go. There is something wrong- I’m just trying to figure out what it is.

50mph in 6th isn’t really telling us anything.

What distance are you driving?

At that distance how long is the drive (minutes vs hours)?

Is the drive city, highway or mix of city/highway?

How many traffic lights do you encounter during that commute? Periods of idle affect fuel consumption.

Is the commute daily and the same route and distance every day, or are we talking a multitude of different routes and sporadic use of vehicle?

Those are important factors in order to help you out a little more.

Here’s more questions to consider:
What is your tire pressure at all 4 corners? Tire pressure will affect mpgs.

What type of fuel are you running (octane)?

What are exterior temps?

 

[Braski]

Sounds about right.

 

[pb29590]

Thanks guys. This is all very good information. I’m going to check the tire pressure as well. I have not looked at that. I have literally driven the car about five times since I bought it. I think I need to run through a couple of full tanks of gas before I can arrive at any other conclusions. It was tuned yesterday by a very reputable local shop and it is not running rich or lean. I will return back to this post after I have gone through a couple of tanks of gas. Thanks again

 

[ugstang17]

Suggest you calculate your MPG the old fashioned way if you have not already. \

Before doing this verify your speedometer matches a GPS that gives you your speed as well. Or reset your trip meter after locking your cruise at 60mph and timing one mile (60mph/60min = 1 minute)Next using mile markers verify the OD to ensure a mile traveled is registered as such on the OD or better yet the trip meter since it does show tenths (cant remember if OD does or not these days). If this is off your estimates MPG will be off. This being off would indicate tire size gear ratios or other related items are not correct. If they are good move on to the next step.

Computers aren't perfect. Fill the car and reset the trip meter. Drive the tank empty. refill and then divide your miles traveled by the total gallons put into the tank on the fillup. do that three-five times and then average the 3-5 MPG values together.

Or you could do as many these days do and accept the MPG estimate on your LCD display as law and gospel when it has never been verified mathematically.

 

[Deleted member 33561]

Listen 16 mpg @ 50 mph is NOT normal in overdrive.. My 18 GT gets about 33 mpg @ 50 mph (assuming) a level road and no wind). I'm also assuming you are talking about computed mpg and not relying on your mpg gauge in the car. You might want to check for a fuel leak somewhere. I get around 18 mpg driving around in my small town. Large city traffic will be worse of course because of stop lights at every intersection. (Just my 2 cents from my own experience.)
I have a JLT CAI with a Lund tune. Every thing else is stock.

 

[NoVaGT]

.....It is registering about 16 MPG overall......I relaize that this mileage is grossly abnormal....

That's not abnormal at all. Not in the least bit.

I wish I got 16MPGs.

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