I bought a 2007 Prius 3 months ago. It was low mileage and for the first two months...GREAT MPG (average city/hwy of low-to-mid-50's). Since then the MPG has dropped to low-to-mid 40's. I have done two oil changes (one regular oil, one synthetic). The air filter has been changed. I have put new tires on the car (recommended by Toyota)...and had a sudden stop to miss a deer. It was immediately after the sudden braking that the MPG began to drop. The more after oil changes/filters...and finally tires. It's frustrating as heck. Ideas?
Check the 12 volt battery. If you do some reading on the Gen II forums, you'll find other accounts of owners resolving MPG drop-offs by replacing their 12V batteries. Optimus makes a very good replacement with battery posts customized for Prii.
You'll also see the change from Summer to Winter gas formulation, and Winter gas can drop your mileage by 10% or more.
Cold weather, winter gas formula and new tires all contribute to lower MPG. Those old tires where giving you better than average MPG. When you change tires (what brand/model are the new tires?) they take some time to break-in. I assume you are running the heater more with winter temps. That causes the ICE to run longer and use more gas.
http://priuschat.com/forums/gen-iii...mileage-gets-worse-in-winter.html#post1411488 What make and model of tires were on the car before and after? Even if you replaced them with identical ones, you could see a drop. See Tire Tech Information - Tire Rolling Resistance Part 3: Changes to Expect When Switching from Worn-Out to New Tires.
Short answer: colder weather. It hurts all cars, it's just more obvious in a Prius. Single best response: pump up the tires. You can go as high as the value shown on the sidewall. Tire pressures should be measured when the car has not been driven for at least an hour, and measured monthly or more often.
