My mileage in my 2010 dropped from 48mpg to 38 mpg...

In a matter of a few weeks. So...

There are many things that could cause this. Tire pressure? Mine is 10% above Toyota's recommended pressure. EGR system causing problems? I checked and cleaned the EGR pipe, the one between the EGR valve and the intake manifold. Nope, it had only a thin film of carbon build up.

OK. It must be my old NiMH battery dying. I made an appointment with a shop that specializes in Prius battery testing and rebuilding. At 12 years old and 142,000+ miles, it seemed the logical cause of the mileage drop.

That same day, my wife mentioned that the car was making a new squeal from the back, as though she had picked up some debris in the rear brake assembly. I pulled the wheel and found terribly worn brake pads on the left rear of the car. Down to metal on metal. Oops! I immediately ordered new pads and rotors. When I went to put them in, I found that one of the caliper slider pins had frozen! It, in turn, caused the brake to be dragging. Ah ha! That explains the bad gas mileage. What should have been a one hour task became much longer, as that pin required a lot of TLC to fix it. Now it's all back together, and mileage is back to normal.

Note that it didn't take much brake drag to cause the problem. I never felt the car pulling to the left, as a serious brake drag would have done.

I don't know how common this is, but I thought you guys would find the story interesting.

 

The right side was just fine. Changed the rotor and pads, though. The internet mechanics are divided as to relubing the caliper pins, so I didn't relube the right side. We'll see how that works out.