I turned rachel in one car seat at 11.5 months, and the other at 13-14 months, I think.
She was right at the height limit rf at 11.5 months. My folks had bought a new van, and (I don't drive, and I ride in their vehicle the most.) I tried & tried to get the car seat in rf, it wouldn't go in properly, and I had a ff only seat already, so I figured that'd be safe correctly installed than rf incorrectly installed.
I used the other seat as a spare. At around 13-14, I had to put her rf, as I was putting it in a seat with no tether anchor. When I went to take her out, I looked at where he head was. (right up at the top of the seat, actually hangin over lsightly) I felt so bad for having her rf'ing.
She was still well under the weight limit, but very much too tall.
If I hadn't had the isse with installation, I'd have used her rf'ing seat till she'd outgrown it by height.
The rf'ing seat was a Cosco Touriva, (I think it's cosco?) I think she might still be under the rf weight limit, (she's 33 lbs now), but by September of this year (so a little over 2) she barely fit in it ff, let alone rf'ing.