Does a dead battery affect mpg?

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  #1  
Old 09-01-16, 10:19 AM
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Does a dead battery affect mpg?

2 weeks ago the battery went completely dead on my 2010 jeep jk shortly after I filled the tank up with gas. It was a day or so later and the dash lights lit up and then went out as I turned the key. Replaced battery.

I normally get 20-20.5 mpg with local driving. I fill the tank between 300-350 miles. This morning the low fuel indicator started flashing at 270 miles and when I filled it up the mileage came to 18.3 mpg. I noticed after replacing the battery that the mpg gauge was a lot lower than normal. It usually lies and says 21 mpg or better, but had dropped to 18 mpg. When I hooked up the battery there was a lot of clicking and strange noises but they stopped after a minute.

Does the computer have to relearn something that would throw the mileage off? It's 3.8 V6 with 60k, runs and starts good, no leaks. Where I live makes it highly unlikely that anyone would have siphoned gas out of it.
 
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Old 09-01-16, 11:33 AM
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Car computer calculations are notoriously inaccurate. Beyond that, average MPG is just that: an average. If your average includes a lot of highway driving it will be higher than an average including mostly local driving. I believe the best way to determine MPG is the old fashioned way: tracking your miles over several tankfuls.
 
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Old 09-01-16, 12:07 PM
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I do track the mileage the old fashion way. I know that read out on the dash is off [personally think it's programmed high to make you think it gets better gas] My local driving is pretty much always the same and has never varied by more than .5 mpg over a tank full .... unless my wife drives my jeep - her lead foot brings the mpg down but she did no driving during the last tank full.
 
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Old 09-01-16, 02:22 PM
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Your car can, and probably did lose some of it's memory. It will build a new memory, based on your driving habits since replacement.

In the future, there are ways to hook up a backup battery to keep the memory module from erasing itself.
 
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Old 09-02-16, 03:28 AM
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But would the computer's memory loss cause a drop in fuel mileage?
 
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Old 09-02-16, 04:02 AM
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No. The actual mileage you get is not affected by a dead battery. However the reading via the computer will start over as Goldstare mentions and a new average will need to be built over time. Theoretically you should be able to disconnect your battery after the car starts. I use to do that years ago, but today's cars are different.
 
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Old 09-02-16, 04:36 AM
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I'm just curious as to why the 2 mpg drop in mileage, odd since the only thing different was the battery. Not going to loose any sleep over it assuming the mileage goes back up with this next tank.
 
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Old 09-02-16, 04:50 AM
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I doubt you fill your gas tank to the same level as the previous time each time you refill. Therefore when you calculate the mpg, the gallons used has a tolerance. My guess is it it at least plus/minus 1 gallon. The less gallons per refill will yield greater variance in your mpg calculation.
 
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Old 09-02-16, 05:20 AM
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I top the tank off every time I fill it up. Unlike my wife's merc, once the pump automatically shuts off you can only add less than a quart. I routinely track my mileage and have never had it drop that low unless my wife had been driving.
 
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Old 09-15-16, 10:06 AM
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Everything's back to normal. I filled my jeep up and my mileage comes to 20.3 mpg
Still curious that the tank with the dead battery lost 2 mpg.
 
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Old 09-18-16, 05:40 AM
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What everybody is saying is most likely the cause, the dead battery caused all the prior milage amounts to be wiped clean and your starting off fresh and with time the values will balance out to the old numbers.
 
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Old 09-18-16, 06:10 AM
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I'm not concerned with what the mpg gauge on the dash says - it always lies! The fact is the tank full of gas that I just filled it with prior to the battery going stone dead, didn't go as far as it normally does. I figure my mpg the old fashion way by dividing the miles by the amount of fuel it took to fill up the tank.
 
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Old 09-18-16, 08:40 AM
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If it's a newer car, the loss of power from the change over from dead to new battery may have have reset the "learning curve" of the transmission and as a result it's like driving a new car for the first time. It needs to learn you driving habits.

I can see the difference between my wife and myself when driving our Ford Focus. Although we are both very similar in our driving habits, she tends to hit the gas harder from a dead stop where as I will ease the throttle and let the car come up to cruising speed as opposed to accelerating to cruising speed. It shows up on the gas mileage.

As far as the accuracy of the dash monitor, mine is pretty much right on compared to the calculated method which I painstakingly keep track. At worse it may only be a mile or two off in the mpg. What I find interesting is the fact that my car meets or exceeds city driving EPA rating of this car but I seldom meet highway or combined EPA rating. But then again we almost exclusively use it for city driving. Lots of stop and go.
 
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Old 09-18-16, 09:16 AM
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Having to relearn shift points makes sense
If I remember correctly the EPA rating for my jeep is 15 city and 18 highway I get considerably better but my wife probably gets worse I have gotten 22-23 mpg on the highway but I seldom get on the highway, most of my driving is under 50 mph but only stop and go once I get to town.
 
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