The first half of this tank of gas, I was averaging 65 mpg. The second half was 100% driving on the freeway. Posted speed limits of 60, 65 and 75. I drove mostly between 60 mph and 65 mph, occasionally hitting 70 mph. This is my final reading. I averaged 60 mpg after 660 miles of driving.
Honestly, it sounds more like some sort of bug in the mpg calculation on the dash than a real mpg just by switching out the 12V battery. Which is why earlier I said you need to look at the odometer / gas pump to calculate on your own.
My guess, would be some sort of voltage monitoring number is used to infer how much fuel is used, and a different than before 12V battery voltage skewed with the calculation (i.e. they assume it would need this and that voltage or power over this much time so you can assume this is the reference to calculate how much fuel is injected). Software bugs happen, and they are often harmless if they were not discovered. So in this case a different battery chemistry they have never seen could cause some calculation mistakes. If they test this in the factory they would have discovered a difference, but if they are not (let's give this whole experiment a benefit of doubt), then that points to one thing:
1) HAH really use a lot of 12V circuits that points to 5mpg of waste in battery charge and drain, really unbelievable, they would have improve that as it is a low hanging fruit
2) You haven't charged the 12V battery the whole time because it is so huge, and that is not a fair test because after a few months you would eventually use up that battery and your 5mpg benefit would be gone. It is probably not real because 5mpg at 12gal tank is still a lot of fuel, a lot of energy, and a lot of electricity. I don't think it is that big of a battery.
3) battery voltage difference causes your car to run in some unusual mode that while gaining efficiency may fail emission? I don't know, if it is not a software calculation bug and your car is really getting 5mpg more from the gas pump reading, you have unlock something that Honda decided not to use (they definitely have done it for sure, as they have the resources to test it), and the reason to not do it is either failing emission or it would cause long term reliability problem (knocking?).
4) The ECU detect some sort of cheat code and unlock an emission fraud scheme just like the VW diesel gate. Honda decided they need one just in case but they are not using it to cheat the test, just kept it in a vault and locked it away.
