britax... roundabout goes to 40 inches 40 pounds.. my kids are still in their roundabouts at ages 3 and 4 1/2.
britax is expensive but the kids say it is comfortable.. and the parents say the straps dont twist and it is easy to install.
we also have the graco nautilus... up to 100 pounds.. but the crotch strrap falls under the child and you have to hunt around under their butt to find it every time you put them in the car.
we get frustrated with the nautilus.-- but it would be the last seat you ever need..
britax makes bigger seats that last longer but they are a lot bigger taking up more back seat room .. heavier to move..