Getting really bad gas mileage in my Prius, what could be the problem?

There s no tune-up on most modern cars, and definitely not on Toyota's hybrids. However, there is a maintenance schedule that calls for the spark plugs to be replaced every 100K miles. I would recommend doing that now to cover that issue - the first rule of troubleshooting is to make everything right and see what symptoms remain. For the same reason this is a good time to do the coolant service, including replacing the thermostat. A thermostat that is stuck open will bring fuel economy down to 25 mpg.

Otherwise, there are the fuel injectors and the throttle body. If you have not been using top tier gasoline regularly those two items don't like it. A bottle of fuel injector cleaner will often get the injectors to behave, but in some cases they have to be replaced. The throttle body gets a lot of soot from the Miller valve timing and you can't use spray cleaner of any sort - the hybrid system will get very mad at you. The cleaner has to be swabbed on and wiped off - I use an Intake Snake on my Prius cars.

One more thing that can cause all sorts of strange behavior, including rapidly dropping fuel economy: the 12V battery may be failing. The DC-DC inverter requires a decent battery to stabilize the 12V that is used for all the computers. If it is more than a few years old this is a good time to have it tested. NAPA carries replacements. Since the posts are undersized they can be hard to find. If you never test it you can get a surprise when you come out one morning and it is stone dead. The 12V battery does not start the engine; the hybrid battery does, so there is little warning.

Finally, it is possible the hybrid battery is failing. That is rare, but you can rule it out by setting the display so you can see the state of charge of the hybrid battery. It is normal for the state of charge to vary a lot, but you are looking for signs the hybrid ECU is trying to equalize the module voltages. That shows up as the state of charge cycling from zero bars to completely full; you should never see zero bars. The engine will run when it normally does not.

The problem is YOU are a gullible idiot who thinks a Prius is economical. About 5 miles on batteries and then you lugs a ton of dead batteries around while a feeble 1.5 l engine struggles with all that.

Hybrids are just a con and you feel for it.

When's the last time you took it in for maintenance?

Failing batteries or electronic battery control..

Check for any trouble codes. Get a tune up.

when was the last major tune up done to the gas engine ??