I attended a meet of the Milwaukee Hybrid Group last weekend and all of the "hypermilers" use some sort of radiator block in the cold weather. This shortens warm-up time so that the car is ready to operate in EV mode sooner. This will improve winter gas mileage but owners still report lower readings in winter due to other factors.

Club members note that it is more noticeable in a hybrid because of the math: a 10% drop in mpg in a car that only gets 15mpg is less noticeable than the same 10% drop in a car that get 40mpg.

All that being said, our winter mileage average never fell below the 26-27mpg range. Now that warmer weather is here we can easily go over 30mpg.