Hellobee Boards

I think you need to do the "pinch test" both with and without the coat. So put her in the seat with the coat and tighten her in the way you normally would. Check to see if you are able to pinch the material of the strap between your fingers (if so it's too loose). Then without altering the straps, take her out, take the coat off and put her back in. Try the pinch test again (based on the tightness of the straps with the coat). If you can pinch fabric between your fingers, the coat is too big to safely wear in the seat because in an accident it would compress.

ETA: Here is a Consumer Reports article with picture diagrams that is clearer than my rambling. http://www.consumerreports.org/cro/news/2012/12/winter-coats-and-car-seats-keeping-your-child-safe-and-warm/index.htm

we usually go sans jacket, if anything, just a fleece/sweatshirt, and then do a blanket!

Fleece is great, it's not going to compress as a down filled jacket would, so we always went with something like that.

Another good test is to put the coat in one of those vacuum sealing bags. If it drastically changes size, it's no good!

I always take LO's coat off in the car. I'll keep a super light fleece jacket on him. The firefighter who installed our car seat scared the living tar out of me. I just hop in the back seat with LO (so we're not both standing in the cold), take the jacket off, strap him in, and put the jacket over him. Same thing in reverse. A huge PITA, but I'm not willing to risk it. I keep hat and mittens on, and he seems to do fine.

When we were in the infant seat, I bought one of those covers and altered it so there was nothing underneath him. I know they make ones that already don't go underneath them, but I didn't know the concern until I'd already bought one

Fleece or sweaters in the car with blankets and I take on and off her real coat. When she was an infant I just covered the seat with blankets but it gets tougher when they're walking around and really need to be bundled up.