The estimated fuel economy for highway is 23 mpg (sorry its easier for me to do miles). You are getting 20.6 that's a difference of 11%.
Low tire pressure and cold weather could account for some of that (off the top of my head 8 psi can account for 12% it might be less), something to check into especially since temps have dropped in the last month. A tire will lose approximately 1 psi for every 10 degrees the temp drops (again based on fahrenhieght, sorry)
New tires can also effect fuel economy. We just had a discussion about an Envoy that lost 17% of its economy after new tires. I assume you drive both cars the same so that's not a factor and I also assume that they have the same engine so that shouldn't be a factor also.