Just wondering. Under the same driving conditions will you get worse mileage if the engine has to charge the batteries?

By the time I hit the expressway in the morning the batteries are about 1/2 way charged. It seems to me based on the mpg gauge I don't get my best mileage for a while and I'm guessing it's because some of the ICE's power is being used to charge the batteries. I know it's not terrain related as everything is pretty flat around here.

Does anyone else find this? Does it make sense (does to me)? Or am I just staring at the gauge too much when I should be looking at the road?

I have the non-NAV version so I don't have the nifty display that shows mpg on a minute by minute basis.