Carseat or Booster for 36Mos Old Who Weighs 40 Lbs.

there's a couple issues here.
the first is - if your 5-pt is rated at 40 lbs - it is usually NOT safe to use for a child larger than that because it is not designed to hold the extra weight.
the second issue is your little guy too tall for the straps? the straps need to be at or above his shoulders or the forces exerted in an accident could cause injury.
lastly - state law in CO is 4 yrs to be out of a 5-pt and into a booster. state law does not have anything on weight (I did a bunch of research because I have the opposite problem - a 5 1/2 yr old in the 85th on height but still only weighs 35 pounds and she outgrew the straps). anyway - I finally got in touch w/someone at the CO state patrol - she said the reason for the 4 years w/the 5 point is that most kids under 4 won't stay in position - meaning sitting up straight, in the middle of the seat with the seatbelt across the shoulder and lap correctly. a 5-pt is always safer because it keeps them in position all the time.
so - if I were you I'd probably look into another 5-pt that will go higher than 40 lbs and also has a lot of room for growth in height and then he can stay in the 5 pt until you feel he's mature enough to move to a booster - maybe long after 4 years old. the only problem is - these tend to be pricey.
as others have said - go have a chat w/the car seat installers in your area (probably the fire department) and good luck.

Ask at www.car-seat.org...man, do those folks know their car seats!

We've got our boys (1 and 4) in a Britax Marathon and a True Fit... both go up to 65 lbs with a 5-pt harness. The Britax was like $270 a few years ago but doesn't convert to a booster; the True Fit does and was regularly priced about $200. (We got one cheaper on clearance.) The height goes way higher on the True Fit than on the Marathon. http://www.learningcurve.com/wps/portal/truefit

You might also look into the Sunshine Kids Radian seats. They're good to either 65 or 80 lbs w/a 5-pt. I have a few friends who have them and LOVE them.
http://www.sunshinekidsbaby.com/

People are also talking a lot about the Graco Nautilus... probably worth checking to see if it might work for you. But I'd for sure go to the first site I listed up top. They do reviews & stuff and if you post your question over there I'm sure you'll get a few great ideas.

Best of luck!

The website for britax has their convertible booster listed with all its specifications. I think it's called frontier. Swedish medical center does inspections again also. they might have other recommendations. good luck.

I would wait until he cannot fit into the five point of what you have right now. It isn't necessarily the weight as much as it is the strength. My daughter who is seven just NOW went into a booster at 46lbs as the Dr said if she can fit in the five point safely, that the straps are where they should be to keep her in it until she isn't okay in it.
That just happened in Sept. My son who is four going on five is 47lbs and I just now put him in a booster.

Five point booster are the safest if they are positioned right. I have both of mine in backless boosters now as they sit in the seat well, have support behind them in our backseat, the main thing on the highbacks is making sure they have head and neck support and if your car doesn't allow on the backseats, for that the high backs are the best.

I just read majority of people do not install them correctly so I take mine to the fire dept to have them check to see if they are okay in the seats they have. Both had 3 in 1 where it could have turned into a high back booster, yours may convert too to that, you remove the buckles and straps and can use the seat itself with the seat belt, but confirm that beforehand. Find out which area local fire dept close to you does carseat inspections, have your son in the seat and let them see if he is okay in it for a while still.

You can call the fire department and ask when the next car seat safety check is and go there. They are trained professionals in this area. Does Britax make boosters? I will be sad to give ours up!!

We have Coscos that have the 5 point harness that can be removed. Both of our boys and both of us do not like them. We just leave them in their seats until they don't fit and then put them in a regular booster. We really like the Graco with the back as it reclines for long trips and allows for a head rest. I really like the Britax, but we've never had the money for it.