Only 29MPG for My Gen 2 Prius?

My 2007 Gen 2 Prius has 270K miles, and it is with me since 2013. It was running well and had average 46MPG (annual average).

However, since last summer, after I drove it in 100F degree weather for 3 days (3 * 12 hours, the hottest three days in middle US) , the MPG became notable lower, and now it only reach 29MPG for today, while the weather is still above freezing point.

I have checked:
1. Tire pressure is not the problem,
2. The spark plugs were replaced, and the throttle was cleaned recently.
3. The engine is consuming oil for about 1.5 QT per 5000 miles
4. Toyota TIS stream shows HV battery Internal Resistance R01~R14 are among 0.024~0.025ohm
5. Sometime I can hear some noise that matching car speed, but that's not bearing noise, as I already replaced two bearings.

Kindly, please share your point of view on why the MPG number is so low now? Anything else I should check?

 

Actually since I got this car in 2013, I always feel the ECU is not so "willing" to use the HV battery power, instead it requests ICE start often. I think this might be worse after that journey, but I don't have quantifiable data.

Since I'm the 2nd owner, I'm not sure whether the dealer ever replaced the battery or not.

I haven't checked the brakes, maybe I need find some video in youtube to do that.

I'm in Seattle now. The journey I mentioned was driving this car from KY to it. During that days, the weather was so hot, and A/C was keeping running but can not reach the desire temperature, I'm wondering whether the HV battery was impacted because of that...

However if it is HV battery issue, looks like the 14 HV blocks are still in same condition, unless I replace them all...

 

The HV battery each block's Internal Resistances and Voltages are the best data you need. Although purchasing a Bluetooth OBD+Andriod APP "Torque Pro" is enough to see these data, you may want to buy Toyota TIS Techstream which is much more powerful.