I recently replaced the flooded cell stock battery with a high performance agm cell. Prior to the replacement I saw the voltage hover around 15 volts for extended periods of time. After replacement the voltage dropped to 13.6 or so. I noticed a mileage increase too. I believe this is related to the new battery.

Over the past four years I noticed a slight drop of average mileage from 26.5 to 24.1. I live in a rural suburb; few traffic lights and light traffic.

the 991 cycles or deactivates the alternator to improve mileage and decrease tailpipe emissions. Seems to me that the new battery needs less charging; less charging means less alternator usage.

Over the past 600 miles the mileage is now at 26.52+ and I have seen mileage highs not seen since the car was new...see attached 37.3 mpg over 20 or so miles.



On a recent highway trip I noticed that the mileage back to old levels

 

I noticed the same voltages anomalies just before and then lower after I replaced mine with an AGM. I don't get as good mileage as you do, however.

ChuckJ

 

need some detailing brushes on your dash.................

 

No offense, OP, but there's no way that your driving conditions were anywhere near precise enough to determine mileage differences due to battery, less alternator use, etc.

 

I thought the 991 already had an AGM battery as OEM. I know my 2015 GTS has an AGM battery.

 

Yeah, I'm not buying that the battery technology is going to have an appreciable impact on gas mileage. I also tend to agree with bittrl1000 that it would be difficult to determine without some precision conditions.

I suppose if you had a battery in very bad chemical shape, that the increased alternator usage might adversely impact fuel economy.

I also wouldn't read too much into the instantaneous voltage that you read on the gauge. At any given moment you are looking at the result of what the charging system is doing to maintain charge on the battery. And within a range it can bounce all over the place depending on current charge state, temperature etc. The charging circuits now days use a lot of different factors as to how the charge. As long as it is in acceptable range there isn't much of a conclusion you can draw.

I can believe that you might see some differences as to how the battery charges (and the resulting terminal voltage) because of the battery technology. But again, drawing conclusions is difficult since charging circuits can be much more sophisticated than mechanical voltage regulators of yesteryear.

In any case, I can't see how battery technology would affect fuel performance. I don't even thing the battery manufactures make that claim.

 

what battery did you buy?

 

Not suggesting that the 37mpg was due to the battery, but I have noticed a 1/2 to 1 mpg increase over a week in my regular commute. Due to battery? I don't know...

dust? Dust will appear in photos that cannot be easily seen.

AGM? Not in a C2

Duracell

 

Maybe your area has switched over from winter gas?

 

It shall remain another mystery.

 

if you are getting better mileage... maybe you are used to the thrill of driving the car and are not leadfooting it all the time like you used to. I have noticed this anomaly myself once I own a car for a few years...

 

got over the 4 second 0-60 fever quite a while ago....it is my belief that the alternator does not engage as frequently as before. I track my mileage and have detected a trend upward...then again, it really does not matter, it's just odd