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Hi. I have a 2010 Prius. I usually get about 600 miles per tank, which is about a week's worth of driving. Now, I'm only getting about 200 miles per tank. I am filling up every other day. No lights are on on the dash and no messages pop up. The car has 84k miles on it. I change the oil regularly. Any thoughts? I'm afraid to bring it to the dealer to get diagnosed until I have some idea what is going on.
 

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First it's a lot colder now in the Northeast, so have you checked your tire pressure lately. Increase the tire pressure if it's low. Also, the ICE needs to run a lot more in cold weather to keep the cabin warm. Even non-hybrids drop in mileage in the cold weather for the same reason. I suspect your drop in mileage is a combination of low tire pressure and the need for the ICE to run more often. I usually run about 5 psi above the Toyota recommended tire pressure.
 

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On our classic 2002 Prius, tire pressure drastically affects mpg's. Our tires are rated at 44 psi max, and I run 44/42 (front/rear). Lower pressure of 36/32 causes mileage drop of 5-8 mpg.

Our weather varies quite a bit in Phoenix, so I check tire pressure every time the weather changes. For example, it was almost 80 degrees today (01/16/14); but a few weeks ago, the daytime highs were in the low- to mid-60's. Also, a weekend trip to Flagstaff may see daytime highs drop another 10-20 degrees.

Also, like Firepa63 says, extreme cold makes ICE run longer to heat engine and passengers. Similarly, extreme heat causes air conditioner to run continuously. Here in the desert southwest, our Prius gets the best fuel economy when the weather is great for driving with the windows down (i.e., no heater and no air conditioner).
 
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