Warming up for 30 seconds or so before driving doesn’t make much of a difference. It’s about a 0.25 MPG drop on average. Letting it idle for longer, like a few minutes, drops the overall MPG by about 2 across the whole tank. I get around 12 MPG in the summer, so that 2 MPG makes a big difference over a full tank.
My MPG went from 18.8 to 18.0 going into winter. I remote start my truck a few times a week for a couple of minutes while getting ready. Not sure how much of that drop is from idling, though.