My DD is 9 years old, weighs 55 pounds, and is approximately 4 feet 4 inches. She rides in highback boosters (Monterey, Vivo, Apex) in our two cars and my dad's SUV. When we travel by plane, we take the back off the Monterey and travel with it as a LBB in the rental car or family cars. When she rides with a good friend of ours, who also happens to be her Girl Scout leader and has an only child about DD's age, they have a LBB for her that I have "approved". She fits well in that vehicle/booster combination and she isn't in a situation where she would fall asleep in the car. The mom was going to be taking her on a GS camping trip and I was planning to put one of our HBBs in her car for that trip. (Trip got canceled due to weather.)
We already have the three HBBs, so she'll stay in those until she outgrows them or can pass the 5 step test. On the random times she rides with someone other than already mentioned, I dismantle the Monterey and she rides in the LBB. If that car isn't available, then I pop the entire Vivo in and everyone just has to deal with it. I get more questions from the parents than my DD or her friends.
So, today we were headed out in the car and DH was working from home and I had time, so I had DD sit in both cars to see if she passed the 5 step test (out of curiosity). Nope. She's really close to having her legs/back in the appropriate places, but the seatbelt doesn't cross her neck/shoulder properly in either car. The lap belt portion is almost correct in one car, but a ways off in the other. My dad is out of town in his SUV, but I am very curious about her fit in other vehicles now.