Tips to revitalize a slowly-decreasing mpg 2006? (160K miles)

I have been driving this vehicle since late 2006 in South Florida, basically the same type of driving since day one (commute, family, store, etc.). The 160K is 95% highway miles, due to my 75 mile daily commute.

Over the years, my average mpg has slowly decreased. Brand new, I could expect 46-47 average. Then it stayed at 45 average for a couple years. But after 4 years, it dropped to 43. Now, at 5 1/2 years old, I'm getting about 41 average.

I follow the same routine-
-Strict 5K oil changes and tire rotation (same dealer each time).
-Mileage drops temporarily when I replace the tires, until they wear in.
-Florida weather has been pretty much the same over the years (hot, humid), so I always run A/C.
-Buy fuel at the same gas stations on my commute.

I'm assuming that it's just wear/tear, and possibly an aging battery. But at 160K, is there anything else I could do to bring back those extra couple mpg's?

Update: Link to post where I installed a new 12v battery

 

Now that I think about it, I believe I still have the factory 12V battery.

 

I just tested the battery using the built-in check on the screen. The result is 11.7v under load, so I'm going to get a new battery asap, and update on whether there is any difference.

I'll also try cleaning the Mass Airflow sensor as well.

Thanks!

Edit: Wow, that battery is almost $200...

 

I had the AC serviced recently. It blows very cold, but I have noticed that it doesn't blow very hard. Sometimes when it's in the 80's outside, the blower seems like it's on low. I have to set the temperature to 70 or lower before the blower kicks up to maximum.

 

Yokohama AS530. So far the best tires I've used, and I always had Michelin Hydroedge before.
I think I have around 15-20K miles on them.

 

Stock size. I don't think I ever had special low resistance tires. Before the Yokohama, I always used Michelin Hydroedge.

The AS530s are also rain tires, so I'm not sure they would make such a difference in resistance.

 

That's a good point. I find the AS530s to have superior grip compared to the Hydroedge, especially on wet pavement. The Michelins were also seemed more fragile, and felt and sounded like riding on overinflated basketballs.
So its probably the AS530s are heavier or bulkier.

Still, like you said, it would only be a contribution to the problem. I started seeing mpg drop while still on the Michelins.

 

Ok, here's an update. Just got back from Toyota and finished installing the new battery. First of all, I have to say that is the most difficult time I've ever had replacing a car battery. Seriously, they made that thing really hard to get out. I think it took me about an hour.

I drove it around for a few miles. Not enough yet to measure any change in mpg (which I will monitor over the week), but I did immediately notice a few improvements.

First of all, the A/C blower now runs much stronger. I actually started getting too cold, because I was used to setting the temp to 70 just to get it to run. Now I had it at 75 and it was freezing.

The engine kicks in more smoothly and quicker. I realize now that the engine still have to "turn over", except at a higher RPM. With the battery being near-dead, it clearly wasn't turning over efficiently.

And also a light bulb came on (in my head). One of the things that had baffled me for years was a weird, whiny sound coming from the engine at random times. In fact here is the link to the forum post, http://priuschat.com/forums/gen-ii-...73-noise-coming-front-car-audio-included.html. It sounded like it was coming from the coolant pump (the thermos reservoir thing), but I could never verify it and everything seemed to be running fine anyway. So I just left it, and eventually it went away.

Today, after I shut off the car, I heard something I haven't heard in a long time, (and never noticed it had stopped); the coolant pump kicked in.

That pump was not running for the last 2 years.

So that probably answers my post from 2 years ago. The battery voltage was too low for the pump to start, causing it to make that "dying" sound. Then it stopped completely. But now I have more questions... why didn't any warning lights come on? Why didn't the battery every get checked in 2 years of service?

Most perplexing is why didn't the battery just die outright? How could it still be starting the car?

Anyway, you guys are awesome. A round of thanks to everyone who helped. I don't know if the mpg has been affected, but I feel like the car works so much better now. And it only cost me $200 and an hour of work.

 

You may be right. I was under the impression the ICE was started by the 12V battery. If it dies, you can't start the car.

As for the AC, some components must be run by the 12V. The blower runs when the car is on but in ACC mode. The condenser doesn't operate in ACC mode though.