I dont think the frontier will go to higher limits than the nautilus.
The only regret I have about the Nautilus is I bought it when my son hit 37 pounds thinking any day now, he won't be able to use his Graco Cargo. But he is STILL under the 40 pounds mark and he is 41 inches tall now. He is turning 5 in a few weeks and begging to go in to the Turbo Booster we already have for his older brother. He sits still and fine for the Turbo Booster. Now I wish I had waited to buy the new carseat when I had to. If I had, I likely would have just put him in the Turbo Booster when he grew out of the CarGo. He has plenty of room in the Nautilus to go for sometime. He only asks for the Turbo Booster because he can fasten the belt himself and he wants to be like big brother. We let him use the Turbo Booster in his dad's car.
The only regret I have about the Nautilus is I bought it when my son hit 37 pounds thinking any day now, he won't be able to use his Graco Cargo. But he is STILL under the 40 pounds mark and he is 41 inches tall now. He is turning 5 in a few weeks and begging to go in to the Turbo Booster we already have for his older brother. He sits still and fine for the Turbo Booster. Now I wish I had waited to buy the new carseat when I had to. If I had, I likely would have just put him in the Turbo Booster when he grew out of the CarGo. He has plenty of room in the Nautilus to go for sometime. He only asks for the Turbo Booster because he can fasten the belt himself and he wants to be like big brother. We let him use the Turbo Booster in his dad's car.