I get mileage like that too. I have no idea how some people get it into the high twenties. If all I did was drive it around town where I live it would be 12-14.

It sucks the gas good.

 

I would hazard a guess (and it's just that) your old battery was influencing the micro resposible for engine control. It may have put your ignition timing in full retard, it may have been running the engine in cold-mode (enriched idle, enriched acceleration, enriched normal mode) possibly many other conditions. But you need to be sure the battery was responsible and it's not a red-herring.
Old batteries do suffer a condition known as sulphation, in which they can "resonate" electronically in the band 2MHz to 6MHz, one of the reasons I fitted a filter capacitor across mine.
All speculation of course! Wouldn't stand up to a serious scientific investigation.
Leedsman.

 

About the only way to tell, if it really mattered, would be to refit the old battery. As if.

 

What the OP was posting for MPG is about right, at least for me around town. I get a little better on the highway, low 20's, but 17 or even a little less is fairly frequent for me. I do drive like a jackazz and stab the throttle all the time, I make no bones about that.

The MPG difference could likely be from the onboard computer resetting the trip odo's - and now he has a much smaller pool of driven miles to evaluate MPG. Wouldn't be unusual for it to display a higher MPG if he had been driving more highway since the reset.

That is just a hypothesis - the OP could clarify by providing details of his trip computer settings before and after the battery swap, and if the trip odo's even reset at all.

 

Better still, fill to brim, note mileage, drive until fairly empty, refill & calculate true mpg.