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I recently been getting a dead/weak battery issue, obviously its just the battery, not the alternator, had the battery tested, it holds a charge for a while, but tends to weaken after 2-3 days. I have another issue possibly related, not sure. My gauges seem to be acting funny. My average mpg says 27.2, but when I filled the tank up, the miles to empty said something weird like 330. when the miles to empty readout would say something like 435 before today 27.2 times 16 gallons is way more than 330. I went into engineering mode and the average mpg said something ridiculous like 20.9 mpg, which is funny, because last time I went into engineering mode the fuel mileage said something like 29.7 mpg. Is it possible that my fuel mileage suddenly dropped, or I'm leaking fuel, or is something glitching? I'm checking for leaks, so far nothing. no check engine lights, no DTC's.
 

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Try tracking your MPG's manually for a few tanks and compare that to what's being displayed in the instrument cluster. Keep in mind that some of the figures you're seeing are estimates/algorithms based on time and when you last reset them. In the 2+ years we've owned our Ecoboost I've never seen an estimated range of over 330 miles for a full tank of gas. We do have an A/T so I'm sure that plays into it some. My wife also has a lead foot and rarely sees real world average gas mileage of more than 23-24MPG's Hwy/Cty combined. We track the gas consumption manually in all of our cars all the time. The Ecoboost is consistently displaying an average MPG which is 2-3 MPG's higher than what we are actually getting.

U.M.

To properly track your MPG's manually you have to remembered to always fill the tank full up each and every time you get gas. if you're one of those people who throw $5 or $10 in it at a time you won't be able to track the MPG's accurately.

If you're not planning on replacing your battery which sounds like it is failing you might consider getting a battery tender to plug it into when you're not driving it.
 

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I recently been getting a dead/weak battery issue, obviously its just the battery, not the alternator, had the battery tested, it holds a charge for a while, but tends to weaken after 2-3 days. I have another issue possibly related, not sure. My gauges seem to be acting funny. My average mpg says 27.2, but when I filled the tank up, the miles to empty said something weird like 330. when the miles to empty readout would say something like 435 before today 27.2 times 16 gallons is way more than 330. I went into engineering mode and the average mpg said something ridiculous like 20.9 mpg, which is funny, because last time I went into engineering mode the fuel mileage said something like 29.7 mpg. Is it possible that my fuel mileage suddenly dropped, or I'm leaking fuel, or is something glitching? I'm checking for leaks, so far nothing. no check engine lights, no DTC's.
If you're still having this problem just charge your battery and use the interstate brand batter. It's a very common problem especially in 2015-2017 mustangs.
 

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2015 Ecoboost Convertible
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It might be your battery or it might be something draining the battery while the car sits. The easiest way to determine the cause is to disconnect the battery negative cable and see if the battery voltage drops after 2 or 3 days. You'll need a multimeter in order to check the battery voltage with it disconnected. If the battery holds a charge after 3 days while disconnected there's a drain on the battery somewhere in the car. Most likely the alternator but it could be one of the circuits. To diagnose the source at this point I'd disconnect the alternator from the battery while reconnecting the battery negative cable. (disconnect the alternator with the battery neg cable disconnected for safety.) If the battery holds voltage after 3 days the problem is the alternator. If not then it's one of the circuits in the car. You'll have to start pulling fuses one at a time to locate the source.
 
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