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Declining Gas Mileage Over Time in a 2006 Prius

When I first got my Toyota Prius in 2006 I was getting over 50mpg in the summer and 46mpg in the winter. Last year I got about 48mpg in the summer
and 43mpg in the winter. This winter I am getting 34mpg. I haven’t changed how I drive, type of gas or tire inflation. The dealer can;t find any problems. I talked to Toyota directly and they said that if the dealer can’t find a problem they can’t help me. My gas mileage has slowly gone down over time and this winter it has decreased a lot. There must be some reason…please, someone, help!!!

I suggest changing the thermostat.
It could be sticky and letting the engine run cooler, but not cool enough to trigger an error code.
It’s inexpensive and IMHO it’s a good idea to change it every 5 years in a daily driven car.

As hybrid drive batteries wear, the gas engine will run a tad more often to recharge. The wear is gradual, so too is the amount of increased gas engine use.
Have a hybrid tech check the drive battery efficiency.

We ALL decline over time…How many miles on your Prius? Ken G. is correct…ALL rechargeable batteries lose a tiny bit of ability every time they are cycled…They die a slow, gradual death, not a sudden death…of course they CAN die a sudden death, but that is not the norm…Also, just small differences in driving habits can have a big effect on mileage…Even things like headlight and heater use effect mileage, winter warm-ups, everything…

Have you by any chance replaced your tires this year with a different brand? This can make a huge difference on a hybrid… Also it’s possible you r gas station changed it’s winter blend this year.otherwise it may be a weakening of your drive battery a others have said

I agree with @gsragtop and @circuitsmith

You should have the special “fuel efficient” tires on your Prius. Not all the chain stores have them in stock.

Change that thermostat.

Here’s another thing to consider . . . check and adjust the valve lash, even if the valves aren’t noisey. Valves that are too tight will definitely cause issues. And you won’t hear tight valves.

If you still have the original 12 volt battery, check it’s voltage, 7 years is a long time.

One other item to consider is that your catalytic converter may be getting weak. Catalysts work only when they are up to temperature. Your car may see that the catalyst is losing efficiency and is keeping the engine running longer to keep the catalyst hot and maintain efficiency. A shop familiar with Priuses and the proper tooling can help identify this.