I have a 2006 Prius I bought from a used dealership about a month ago. It had 169k miles on it. He said he had replaced the hybrid battery with a refurbished one about a month before I bought it and he replaced the 12V right after I bought it because I was having an issue with the electric park (which seems to have fixed that issue). It has about about 172k miles now. Originally I was up to 53.5 mpg and going up every day only running the a/c on and off as needed, but now it has been close to 100 every day and I'm running the air all day and I'm down to 50.4 mpg in about a weeks time and dropping daily. I drive it about 120 miles in town a day and I do sometimes park and idle for awhile at times. Does this seem like a normal amount of lost fuel economy? It seems like lot to me.
If you had a 25 mpg vehicle, that 3.1 mpg loss would be a lot, like 0.68 gallon per day. But with starting from above 50 mpg, that is costing you only 0.14 gallon per day. The non-linear math makes it seem worse than it really is on high-MPG vehicles. If we figured this in the style of European fuel consumption ratings (but converted to English units instead of liters and kms), you'd see consumption rise from 1.87 to 1.98 gallons/100 miles, though nearly everyone would round that to a single decimal place. For people coming from cars and SUVs that traditionally burned 5 to 6 gallons per 100 miles, that 0.1 gal/100mi increase for AC would seem like almost nothing.
My personal opinion is that any Gen 2 Prius able to get 45+ MPG this many years after manufacture is doing just fine. Even more so in that heat.
The sweet spot for most Prius mpg is 10 or 15° F below the temps you mentioned, which is probably why the mpg was raising steadily before the extreme heat hit your area. The A/C on the Regular Prius is pretty efficient, until it has to run consistently to keep the cabin at a reasonable temp. ie: How hot does the cabin get when just sitting in the sun? 150° maybe? Same thing happens to mpg up north when the cold weather arrives, although the reasons are not as obvious why if not driving in it every day for months at a time.
