Battery replacement = better gas mileage?

Just replaced the battery in my 2004 Infiniti FX35 (before the battery died… it just wasn’t as crisp during startups and the voltage had fallen to 12.19 volts). During the past 4 days I have been getting about 2 mpg better gas mileage!?! Is there something that resets during a time with no power (kinda like the radio station presets)? Or is there another reason or theory?

Your sample size is too small to be relevant.

Nope. Not that would improve mileage. Besides, 4 days is too short a timeframe to be making any comparisons…unless you’re driving 500 miles a day.

…not for a theory…

Yes, the PCM leaned data does normally get erased when you disconnect the battery. It should relearn it all in a few days or less. However, I would expect that would slightly reduce the mileage for a short time.
Has the weather or something else changed in the last four days too?

12.9 volts on a sitting battery is pretty good. (Sitting being defined as more than a few minutes after the engine was shut down.) 12.9 would indicate and alternator or cable problem if that was taken while the engine was running.

In general…
Engine running = 14 to 14.5 volts
Right after shutdown = 13.8 volts
Several minutes later = 12.6 to 12.7 volts

…it is my same route 20 mile route every day… and I punch the reset at the beginning, so I have 8 legs at 20.2 rather than 18.0… Go figure?

Interesting… Actually the overnight voltage of the old battery was 12.19 (12.2 if you round up)… the table on this page:

suggests that the battery is less than half “state of charge” and the engine running was 14.5 which suggests that the alternator was fine… It seems also that some of the driving characteristics are different (throttle is more sensitive at start, shift patterns are at a little higher revs)… I will take a longer distance look at the mileage. Thanks for your insight, J

PS the weather is the same, but would there be a difference in winter/summer blend of gas? … in California? I buy at the same gas station and filled up about the same time as the battery was switched…

Replacing your battery did not improve your gas mileage, throttle response or shift points. There isn’t anything magical about replacing a battery and nothing is different about your car and the millions of others on the road. But I’m glad changing the battery has given you so much satisfaction.

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[i] During the past 4 days I have been getting about 2 mpg better gas mileage![/i] 

 May guess that you are using an on-board milage computer that will report your mileage for the last trip, day etc.   My guess is when you replaced the battery it re-set the mileage computer and it is reporting based only in the data it has seen since the battery was replaced.  In time it likely will work out to what you what you had been getting.  

It may also have changed other settings of the car's computer, Then the computers can relearn settings it maybe reporting correctly. 

The only real way to get exact mileage is to do the math a Miles/Gallons.  Again you need to measure several fillups in a row 

Time will tell.

Good Luck

It is a possibility the system was being drained by trying to charge a dead battery, and not providing as much kick in the spark, but I like the computer reset.

Sorry, I misread your 12.19 as 12.9 You are correct, 12.19 is too low.

Is it possible that low voltage was causing you computer to get lower than correct voltage readings from the sensors that it uses to compute your mileage?

Thanks, I had not thought that the actual MPG calculation might have memory/history… I will do a few tank measure for greater accuracy… J

The two fillups since the battery change produced the same MpG as before (19.6 and 18.7)

As others have said, the only way to accurately measure mileage is to fill the car, run it until you’ve used some gas–probably half a tank or more would be best, then fill it up again and divide the miles driven by the number of gallons it took to fill it.

The MPG gauge on your car isn’t going to be as accurate as doing it the ‘hard way’, no matter how good it appears to be.

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The engine runs off the alternator, not the battery. Replacing the battery would improve starting but once started, the alternator provides needed electricity for all the electrical components. The only time you’d run off the battery is if the voltage regulator and/or alternator were bad. Old cars ran off the battery and the generator recharged the battery. That stopped in the mid '60s. I’ve actually started cars then removed the battery and everything was fine. Don’t try that now, you’ll blow sensitive electronics.

Sounds like you may have slightly altered your driving habits.

Typically our posts about battery replacemt involve some sort of negative event happening not better mileage as you suggest.

The idea about low voltage to the computer does not pass muster as there is a range of supply voltages allowed and then internal to the device these voltages are stabilized into the voltages the computer uses (typicaly + - 5v for the integrated circuits)

I agree with you until you get to the part where you say “the only time you would run off the battery is when the voltage regulator or alternator are bad” not quite true. If system load exceeds alternator capacity the battery will step in to make up the difference. The alternator does not have to be"bad" for this to occour.

Let me summarize this small adventure… I get the same mileage as before the battery change, but the calculator that displays the ongoing MPG seems to read high (shows 25+ mpg on flat highway driving where it rarely displayed 22+ plus before along the same longer haul trip)… So something electronic has been altered or reset, all the mechanics are identical…

Thanks for all your comments! J