Yes, the Marathon will use the harness to 65 lb...
You can view it on the Britax website (
www.britaxusa.com/ ). From the Britax FAQ:
What are the heights of each of the harness slot positions for the Roundabout and the New Marathon?
The following measurements have been taken from the Roundabout and the Marathon without the cover or foam padding. Due to the fact that different covers have different consistencies, these measurements will change when a cover is placed on the seat.
These figures are measurments from the seat base to each harness slot.
Roundabout
Lower Slot 11 1/4"
Middle Slot 14 1/4"
Top Slot 16 1/2"
Marathon
Lower Slot 11 1/4"
First Middle Slot 13 1/2"
Second Middle Slot 15 3/4"
Top Slot 18"
Due to changes in models and materials, measurements are subject to change without notice.
Keep in mind that a 4 year old grows much more slowly than a 4 month old. If, for example, he has a full 1" until his ears are even with the top of the seat, his full height would have to grow by ~2" (assuming the growth were evenly distributed between his legs and his torso, which isn't necessarily a safe assumption to make). I've had myself in a panic several times over the past year, thinking that my son would outgrow this seat "soon". Well, he's still got room left, and I'm very glad that I left my options open, rather than buying something last summer.
Unless you really just do not have access to other car seats where you live, I still think you are better off with a wait and see approach. In fact, I *do* live where mail order is my only option for most new seats (we live on a military base overseas, and the selection at the base store is pitiful), and while I have scouted out my options in case my son hits a sudden, immense growth spurt, I have yet to purchase anything, as he really does still fit well in his current seat.
If you want a list of alternatives that I would consider either for the interim (if your son were to outgrow his seat tomorrow) or as long term replacements, I do have a list.
Keep in mind that most of these seats will not live up to the ease of use that you are used to with your Roundabout. However, if you try them out, and they do fit your son with room to grow, any of them would be an acceptable next step.
Features to look for in a combo seat are:
1) Front adjuster (some seats make you uninstall to adjust the harness)
2) Higher harness slots so the seat isn't outgrown by height significantly before 40 lb (Cosco/Safety First/Eddie Bauer seats have notoriously low harness slots -- lower than the slots on your Roundabout!)
The combo/booster seats that I would consider (in no particular order!) are:
1) Graco Grand CarGo. (
www.gracobaby.com/Section...TypeID=325 ) The harness adjuster is on the harness itself, which I think is easier to use, but other people disagree. Try it out first or be prepared to return it if you don't like it.
2) Graco Ultra CarGo (
www.gracobaby.com/Section...TypeID=354 ). Almost identical to the Grand CarGo, but the adjuster is on the bottom, front of the seat. Some people prefer this adjuster over the one on the Grand version.
3) Next Step DX/MX (
www.centuryproducts.com/S...egoryID=19 ). The DX & MX versions are very similar to each other. I'm not sure what the differences are, but as long as you get one with a front adjuster, it's a fine option.
4) Evenflo Express (
www.evenflo.com/ep/on_go/...t.phtml?nf ). This seat comes in a Comfort Touch (more padded & a little more expensive) and a regular ("bare bones" & less expensive) version. It has an adjuster on the harness like the Graco Grand CarGo, but the new ones have a push button release, which makes it easier to loosen the straps.
5) Evenflo Apollo (
www.evenflo.com/ep/on_go/apollo.phtml?nf ). Of all the seats listed, this is the one with significantly more features. If I needed a combo seat, this would be at the top of my list (assuming it fits my car -- I have a difficult to fit vehicle). First, this seat is lined with "energy absorbing foam" (like you find in a bike helmet or your Roundabout); it is the only combo booster in the US to have it. This means instead of striking hard plastic in a crash, your child's head will impact on the foam. It has a knob harness adjuster that you turn to adjust the harness tightness. I've heard that it takes a little getting used to, but that it is very nice (easier to use than traditional adjusters) once you figure it out. It has a "memory harness", which means you don't have to re-adjust the harness every time you use it. The Comfort Touch version is very well padded. I don't know if it comes in a non-CT version or not.
I encourage you to try them out and get the one that you personally like best. Look at how they fit your child, how they fit your vehicle (especially if it needs to fit next to one/two other seat(s)), and how much growing room is left (remember, your child should stay harnessed until at 40 lb if possible). Try the harness adjusters to see which ones are easiest for *you* to use. Budget-wise, these seats generally run $60 to $100 (watch for sales!).
Another option is the Britax Laptop. It runs $65 (more or less). Darren posted a little about it on the Reviews board (
pub59.ezboard.com/fcarsaf...ID=3.topic ). This is definitely one to try before you buy. My son didn't like it when we tried it out. It does make an easy to use & compact spare seat, even if you decide to get a true booster, combo seat, or Marathon later.