I have a 2006 with 161K. Reading on screen was 34-36mpg. Didn't have heat and spark plugs were bad...Replaced spark plugs with OEM and coolant control valve (OEM) to fix heat. MPG shot up to 42mpg on drive home. Then next day back to 35mpg on screen. This is my wife's car. I don't drive it much. I think it is still getting 300 miles a tank --- which is quite low. Never did hand calculation. It has Michelin tires (not Defenders can't remember brand). Stock size. Did 42psi front and 40psi rear as many advise on this forum. I am putting KYB struts in the front today as they are original and hit bumps kinda hard...Not sure if that would affect MPG. Has no codes and starts up every time...Everything functions well (except the other day navigation started to not work can't remember message I'm receiving on screen). You would think it's a sign that the hybrid battery is about to explode but I don't know. If you don't look at MPG it runs just like all the other gen 2s. I checked the battery compartment and it has a yellow top Optima so it must have been replaced recently (I just got this Prius a couple months ago). From Texas so maybe the heat negatively impacted vehicle somehow.
Check to see that the engine oil isn't overfilled Check engine air filter Check hybrid battery fan (this one requires disassembly) as it can be clogged with dust. (This video is for the 3rd gen but look at #21 - Cleaning HV Battery Fan Nutz About Bolts Prius Maintenance Videos | PriusChat) That's all I can think of at the moment. There's always the possibility of a battery that's on its way out. Our Gen 2 was giving us about 35mpg (but we were doing short trips too) for the last couple of years of its life.
Replace the Inverter coolant. Probably needs a new 12 volt battery. Easy test I recommend so your wife does not get stranded.: Get a volt meter. With the car OFF turn on the headlights and measure the DC voltage across the 2 battery jump points under the hood in the black plastic fuse box. Report that dc voltage back here. I'm guessing 10.9 volts. Most accurate reading is first thing in the morning before you start the car. And the hybrid battery is getting very tired. You have a window to rejuvenate it though as it has not thrown codes yet. Read the posts about Prolong Battery chargers on this site.
Depending on how it is being driven it might not be THAT bad. Check the suggestions above but if she is only driving it a mile or two at a time "short-hopping" around town the mileage is going to be in the 30s. The car is meant to be driven over longer stretches than that. During the school year we get tank averages in the mid-30s (worst tank is 33.5) when the car is stuck here around the house for only market trips, running kids to and from the local school, etc. but it still is comfortably in the 40s when it sees freeway time. In fact, after seeing multiple tanks in the 30s before our summer trip we had tanks of 54 and 50 mpg at 70-75 mph driving cross country in July. The entire trip averaged over 44.
The CCV has ZERO affect on engine temperature/cabin heat, it's only job in life is to STORE hot coolant when the engine is off to reduce warm-up time. That clearly suggests driving style/habits are a major factor. It will be WORSE with those Does the HV voltage get into and stay in the GREEN, or does it spend all of the time in the lower 1/2 of the scale? If you see green regularly and can drive in EV mode (low speed w/light throttle on level ground) for more than a mile, the HV pack is ok.
Mine gets 47mpg in the summer. Around 38mpg in the worst parts of winter - if it's sitting outside overnight, if it's very cold out, if I'm only doing short trips where it doesn't get warmed up. My understanding is that the low mileage has to do with the vehicle not using the battery until things have warmed up. So 35mpg definitely sounds like it could be the battery being on it's last legs.
