I've read other posts, but am not sure how to best proceed. On the highway I do get around 20-21 mpg, as advertised. However, when I fill up and then run errands and just commute to and from work, which is only a few miles, my mileage is absolutely horrible. I'm not gunning it at every light and I'm not stopped for long periods of time. My last fill up had an average 13 mpg calculated (it said 13.5 on the dash). 2013 RTL with 103k miles. I can't imagine it should be this bad? I'm not sure I have averaged over 15mpg other than an all highway trip. Is there something that could be killing my mileage at idle or lower speeds? When I fill up and only have 10-15 miles on it, I can watch the calculated mpg drop multiple points while I sit at a light. That just doesn't seem right?