I read some posts where people thought there hybrid was loosing mpg as they got to the 15,000 mile range. I thought the same thing ,but I drove to las vegas 4 weeks ago and in the last 4 weeks here
my tank average went way up to 29 mpg from 20-25 in cold minnesota. I guess hybrids hate cold weather...JOE

 

The HiHy gets much if not most of it's fuel savings from the ability to shut/slow the ICE down. Unfortunately, if there is demand, it must burn gas for heat.

 

It is not only for heat. It is not able to use electric power efficiently in cold weather. The batter is always close to full. No matter how hard you try, the car won't use it. I am getting 20mpg at around 27 F and 24 at 32 F. It is a design issue. Above 40, I get descent mpgs (28).