We've had our Prius, a 2007, since June. It initially got 47 mpg routinely, but around December it dropped nearly overnight to 38.8 mpg. We live in western Oregon, which simply never gets THAT cold in winter -- average lows are in the 40s. Our service guy at the local Toyota dealership says the lower mileage is due to one of two things: using the heater more, and buying our gas at Costco. So we started buying gas where he recommended (Chevron or Texaco) and on 3 tanks of non-Costco gas we averaged ... 38.9 mpg. So it's not cheap gas. And I refuse to believe that running the heater at 70-72 degrees has dropped our mileage from 47 to 39. ALL our driving is around town. We don't jack rabbit start; we're really conservative drivers. We don't haul heavy stuff. Our tire pressure is fine, in fact we had the 45,000 service done a bit early in order to try and find the gas mileage problem--so everything should be fine. What could the techs possibly have missed? And if the battery has issues, would that cause the sudden, dramatic decrease in mileage? How do we find if the battery is a dud? It's a certified used Prius from the dealer so has an extended warranty....
Start here: http://priuschat.com/forums/newbie-forum/73400-weird-stuff-happening-mpgs-dropping-test-battery.html Go back to Costco for your gas. The dealer was likely just trying to get you to use a different station just in case there was a problem with your old one.
Using the heater can cause the ICE to run to provide cabin heat, if the ICE is cold. See my detailed explanation at http://priuschat.com/forums/newbie-forum/91431-one-3.html#post1287385. Colder weather will hurt your mileage. See http://priuschat.com/forums/gen-ii-...-want-mpg-help-read-first-20.html#post1295931. How long are each of these trips in time and length? Has the commute changed vs. when you were getting higher mileage?
Start with tyre pressures and health of the 12v battery, those two things are the cause of most complaints. If still unhappy take the car for a spin: warm up the ICE with 5 minutes of driving, then drive a loop with the MFD MPG meter reset from zero, preferably on a freeway around 60 mph on a day with little wind. MPG below 45 would be worthy of further investigation.
You are right, the cabin heater alone won't do that. Winter gas, engine heat loss to the cold winter air, and foul winter weather (e.g. rain on the roads) are also necessary to cause that much loss. But Costco gas? The tech is just bluffing on that one. My Prius is showing a 7 mpg spread between summer and winter, almost the same spread as yours. But I don't turn the heater above 67, nor on at all the first couple miles unless the windshield starts obscuring. And my driving mix includes a good chunk of highway miles. This doesn't eliminate the possibility of problems, but the summer-winter spread by itself does not indicate a problem. This graph shows my seasonal MPG:
The combined effect of heavy heater use and low tire pressures can explain this. Check tire pressures at least monthly. You can safely inflate any tire up to the maximum pressure stated on the side of the tire, which noticeably helps fuel economy (but also affects ride and handling).
