The car's gas mileage dropped from about 45 mpg (19 kmpl) to about 35 mpg (15 kmpl) - I don't know why unless it's the ethanol in the gas or less efficient tires.
aging battery ?
would be normal, something that is rarely discussed, while a 3-4 year old battery is way less efficient as well as less capacity than a new one.
most discussions everywhere are based on data with new batteries as well as average conditions.
i think this is good, because if we would bring that battery replacement costs as well as aging battery efficiency loss on the table at large, even less people would switch to EVs, but i'm sure we're in for ugly surprises in that aspect (costs for people who have a tight budget will hurt them)
for those who want to get an idea, watch your smartphone's battery over the years, at least in case you keep them for a few years like i do.
i have 6 phones in use (various countries SIM for business are the main reason) and replace one of them per year, means my oldest phone in use is 5 years old and that allows to keep a good record about battery life development of similar tech under almost identical conditions (same user, same habits, same climate, same software [rooted and unified]
the difference in the charges they hold, as well as voltage stability is tremendous. some phones with a mediocre electrical system like those from i.e. motorola, would even start to shut down below 50% battery capacity after 3-4 years due to voltage fluctuations etc.
keep an eye on it, you'll be surprise and it's well worth to consider a battery replacement for
equipment, be it car or phone, that else is in top condition. at least i usually replace the battery once before replacing the oldest devices after 5-7 years.