Suddenly bad gas mileage

Depends on highway speed, 42 MPG can be good for 75 MPH. Around 55-60 MPH is ideal. Now we are into cold weather and this always takes a huge hit in MPG. Check your 12v and check brakes sometimes they grab and the wheels do not spin freely.

What kind of tires you got on there?

 

There's a couple of probable causes there, which is good, ie: hopefully not due to malfunction. You're now doing a lot more high speed driving; the Prius shines at lower speeds, around town. Maybe you can change your route?

Also, one tire 10 lb under can't be helping. If it continues to lose pressure something's going on.

 

I think they switched 0ver t winter gas, which gets w0rse mpg.

 

I doubt it's the route, I've been getting over 40 in the v for some time now, and most of my mileage has been hilly interstate.

Lately the temperatures have been driven way down. We had 20's this past weekend, and the winds are picking up quite a bit. It hasn't driven my mileage down too far, but it's getting very hard to keep it up. One thing that's almost always true this time of year is that tire pressure gets driven down with the temps.

I'll bet mine are down 3-4 psi by now. Getting ready for snow tire mounting too.

 

One (of many) modes of failure for the 12 volt battery involves one dead cell, which makes the maximum voltage 10 volts.
The inverter will constantly attempt to 'charge' the battery back to 12 volts, consuming a lot of power.

Testing the 12 volt battery after it has set for several hours may help diagnose your issue.

 

I'm having a similar problem with my Gen 2 (2008). Temps are colder now which explains some of it, but mileage is right around 42.5 with last three tanks. I've kept an eye on tire pressure - was down about 2-3 lbs. last weekend when I checked. I tested my 12-volt battery last year with borderline results. I mentioned it to the service guy at the dealer, but he said not to worry that the battery was fine at the last visit (about 2 months ago). It seems like the engine is running more even though I keep the interior temp set to 66. Not sure, but I think I'm going to go ahead and replace the 12-volt. It is the original and the car has now been relegated to the driveway - the truck wanted its spot in the garage again. Need to expand the garage a couple of more bays I guess.

The good news is that the Gen 3 continues to rock the gas mileage. 51, 50.2 and 50.3 on the last three tanks. Both cars follow the same route when they are driven as we swap out the cars regularly.

 

Might be thermostat failing?

Typically they prevent coolant flowing to the radiator whenever coolant temp drops below some threshold. But if they fail it could cause lower than normal coolant temp, which in turn runs the engine more, and less efficiently.

If you can find a way to monitor coolant temps, freeway driving is one time you'd see unusually low temps with a failing thermostat, due to high speeds and efficient cooling.