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Discussion Starter · #1 ·
Hi,
I can see, that generally people are happy with their Mpg, that oscillates ~25-30 mixed driving. But part of Mustang EB owners, including me, suffers from very low Mpg. Mine is ~15 Mpg (15l/100km), sometimes even lower. And this is acctualy calculated between fueling. The difference is drastic, and I'm pretty sure something is wrong. I read that people drive only in S mode, and have higher Mpg. I try to treat gas pedal gently, even just to check how far on a gallon I could go.

I've got 2015 Mustang US version with 2018 engine (swapped)
6 speed auto
still pretty wide winter tires (285 on the back).
Magnaflow catback
no other mods
I guess the car runs ok, spark plugs and oil were changed few thousend miles ago (~3), although I recentelly experienced lack of power, once. No error codes, tho. AFR ~14


Has anyone experienced low Mpg and found out what was going on? I'm 99% sure it's not about driving habits, because I really tried to keep the Mpg high, it's just not possible.
Idk where to start... I'm thinking of having it tuned (there's a guy that knows everything about this cars - but he's pretty far from where I live), maybe it needs to be calibrated after an engine swap.
 

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A few things I found when trying to get the most MPG
  • Short trips kill gas mileage.
  • Motorway (freeway) driving likes to increase the MPG
  • A well maintained car will increase MPG.

Are you calculating it yourself or letting the computer do it?
 

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"Loss of power"
That's where to start, check everything and confirm all connections are correct and secure.
When you swapped engines, was it just the long block or did you transfer the OEM mechanical components and electronics with the new motor?
I don't know if the early Ecoboost Mustang is tuned the same as the 2018?? But I have been told that there were changes made in 2018, the torque rating increased and the automatic transmission is a 10 speed...!??
 

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Make sure your tire pressure is where it should be also. Very low tires can also kill mileage.
 
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What's your mpg at a steady 70 mph?
Those wide rear tires will reduce your mpg some. Do you have the Performance Pack? That hurts mpg a little too. Is your car an automatic? Driving in S will reduce, not increase mpg.
 

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  • Short trips kill gas mileage.
This, in my opinion, it the most important factor. A lot of short trips (less than 10 minutes) in city driving, especially on a cold engine are the absolute worst for fuel economy. The PCM keeps the mixture very rich until the engine achieves normal operating temperature, and city driving requires constant acceleration to get the car up to speed between stops. That means you are accelerating using more fuel than normal.

I observed this first-hand a few years back when I noticed my fuel economy dropping mysteriously. My typical usage is a short 2-mile drive to the interstate, a 35-mile cruise at 75-80 mph to my work exit, then into the work parking lot. Under those conditions I averaged between 30-34 mpg over a tank of gas, depending on the time of year. Mileage was lower in colder months. Then suddenly, my mileage dipped down to the 26-28 range. What changed? I stopped taking my lunch to work with me, and was using the car to get lunch--a short drive in city traffic on a cold engine. That was a 15% hit in fuel economy over a tankful of gas.

Do the math: I usually fill up the tank after 400 or so miles of driving. My lunch run was typically less than 5 miles round trip, which is 25 miles or so over a week. 25 is six percent of 400, so fuel economy dropped 15% by driving short trips for six percent of the time. Short trips on a cold engine are the worst conditions for fuel economy.
 

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Discussion Starter · #8 ·
"Loss of power"
That's where to start, check everything and confirm all connections are correct and secure.
When you swapped engines, was it just the long block or did you transfer the OEM mechanical components and electronics with the new motor?
I don't know if the early Ecoboost Mustang is tuned the same as the 2018?? But I have been told that there were changes made in 2018, the torque rating increased and the automatic transmission is a 10 speed...!??
The block only has been changed. "Loss of power" only happened once, and I think everything is fine since. I did a trip yesterday, and I gave the car really hard time. Seemed ok.
I know there is difference in power between 2015 and 2018 models. But everythink works fine, maybe it's an engine from previous model year, I'm not sure.
 

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Discussion Starter · #10 ·
What's your mpg at a steady 70 mph?
Those wide rear tires will reduce your mpg some. Do you have the Performance Pack? That hurts mpg a little too. Is your car an automatic? Driving in S will reduce, not increase mpg.
I guess this is where I should start - tires. No, I dont have PP, it's an 6 speed auto.
I know it's worse for mpg to drive in S. I meant that even people that drive in S on everyday basis post on forum lower mpg results.
 

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Discussion Starter · #11 ·
This, in my opinion, it the most important factor. A lot of short trips (less than 10 minutes) in city driving, especially on a cold engine are the absolute worst for fuel economy. The PCM keeps the mixture very rich until the engine achieves normal operating temperature, and city driving requires constant acceleration to get the car up to speed between stops. That means you are accelerating using more fuel than normal.

I observed this first-hand a few years back when I noticed my fuel economy dropping mysteriously. My typical usage is a short 2-mile drive to the interstate, a 35-mile cruise at 75-80 mph to my work exit, then into the work parking lot. Under those conditions I averaged between 30-34 mpg over a tank of gas, depending on the time of year. Mileage was lower in colder months. Then suddenly, my mileage dipped down to the 26-28 range. What changed? I stopped taking my lunch to work with me, and was using the car to get lunch--a short drive in city traffic on a cold engine. That was a 15% hit in fuel economy over a tankful of gas.

Do the math: I usually fill up the tank after 400 or so miles of driving. My lunch run was typically less than 5 miles round trip, which is 25 miles or so over a week. 25 is six percent of 400, so fuel economy dropped 15% by driving short trips for six percent of the time. Short trips on a cold engine are the worst conditions for fuel economy.
I consider it (city driving) as the biggest factor. But still avareging 15 mpg when driving resonably is very bad even in the city, don't you think?
 

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I consider it (city driving) as the biggest factor. But still avareging 15 mpg when driving resonably is very bad even in the city, don't you think?
Yes! I get 5 mpg at WOT on the drag strip...
When we 1st purchased our Mustang, I'd average 32 mpg and my wife (with her lead foot) would get 26 mpg to 28 mpg.
If you're running Wide Open Throttle 50% of the time, then 15 mpg is understandable, but if you're doing normal city driving and getting 15 mpg, then you have a problem.
 

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I guess this is where I should start - tires. No, I dont have PP, it's an 6 speed auto.
I know it's worse for mpg to drive in S. I meant that even people that drive in S on everyday basis post on forum lower mpg results.
What mpg does the car get, when driven on the highway, at a steady 70 mph? There are too many variables to try to compare city mpg. What rear gears do you have?
 

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Discussion Starter · #16 ·
What mpg does the car get, when driven on the highway, at a steady 70 mph? There are too many variables to try to compare city mpg. What rear gears do you have?
It gets ~30 mpg on highway. But it takes just single overtaking and it drops significantly.
Unfortunately I don't know my rear gear ratio.
 

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I consider it (city driving) as the biggest factor. But still avareging 15 mpg when driving resonably is very bad even in the city, don't you think?
I mentioned two factors: city driving (short trips) and operating temperature. If your commute is an hour of driving in city traffic then I would agree that 15 mpg is suspect. If your commute is 20 minutes of driving in city traffic, then you are driving the majority of the time without the engine at normal operating temperature, especially in the winter in colder climates. In that case I would not be surprised at average of 15 mpg. Regarding gear ratio, unless your car is equipped with the performance package (or has been modified) it should have a 3.35 rear end.
 

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It gets ~30 mpg on highway. But it takes just single overtaking and it drops significantly.
Unfortunately I don't know my rear gear ratio.
At 70 mph, in good conditions, I always get around 35 mpg, calculated. My trip meter is typically 1-2 mpg higher than actual mpg. By good conditions, I mean fairly flat terrain, no rain, no A/C, and light to moderate traffic. My car is the best combo for mpg. It's a light base model, non PP, A10, 3.15 gears, and 235 width tires. It'll get 40 mpg at 60 mph. What's your Air Fuel Ratio at a steady speed and also while accelerating?
 

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It gets ~30 mpg on highway. But it takes just single overtaking and it drops significantly.
Unfortunately I don't know my rear gear ratio.
The 3.15 axle ratio is code YY. You can check your axle code on the sticker inside the driver's door jamb. Your axle ratio is also listed on the car's original window sticker.
 
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