It sounds like you want a new convertible seat. Here's a list compiled for a car safety forum that I frequent:
(United States)
Convertible Seats
Britax Marathon, Decathalon, or Boulevard (MA/DC/BV). 17 in top slots, RF 5-33lbs, FF up to 65lbs in 5 point harness. Awesome seats, allow for rf tethering, has lockoffs to replace locking clips, very easy installs, many bells and whistles that make them easier to use correctly every time, lasts most kids to age 5-6y/o. 220+ Check the FAQ sticky thread for a description of the three.
Fisher Price Safe Voyage Deluxe (FPSVD).17 inch top slots, RF 5-33lbs, FF up to 55lbs in a 5 point harness. Made by Britax, sold under the FP name- basically a stripped down Britax Marathon. Great seat, awesome price $129 (regular 199$), and will last most kids to age 5-6 y/o.
DISCONTINUED DUE TO POOR SALES-- As of right now Britax is still manufacturing them & new dates have been shipping.
New Evenflo Triumph Advanced RF to 35lbs; FF to 50 lbs. Max harness height of 17". The new Triumph has the infinite harness adjuster too.. no rethreading the harness! Much nicer than the old Triumph! Much taller shell than the old Triumph too! Priced at 150$ VERY new seat so not in many stores yet I don’t think. We think it will be small enough for newborns!
Sunshine Kids Radian. 18 in top slots, RF 5-33lbs, FF up to 65lbs in 5 point harness. Great seat, only second to Britax because it has some compatibility issues in some vehicles when installed with seatbelt. Really good seat otherwise, allows for RF tethering, lasts most kids to 6-7y/o. 8yr lifespan (most have a 6yr) $175+
www.skjp.com has the manual available for viewing.
Recaro Como & Signo. Recaro makes seats for NASCAR and has made car seats for the European market and a few for the US market for years. RF to 35lbs, forward face to 70lbs, and has 19" top slots. Bottom slots of 9" so not ideal for newborns. Allows for rear facing tether!
The outside width of the seat at its widest point is approx. 19 ½” to 20”.
The Como & the Signo are basically the same shell the biggest difference is that the Signo has an infinitely adjustable harness (where the Como has to be re-threaded) and the Signo's latch straps are on a bar - similar to Britax (on all their models) - so you don't have to re-thread the latch belt if you're going from rear facing to forward facing.
Como is $249; Signo is $289
Como is set at 2nd week of August
Signo is 3rd week of September
And lower cost Convertibles we love... (For primary or secondary cars... they are still GREAT seats)
Cosco Scenera $40
also a good travel seat 
5pt harness version.. Very basic no frills seat. Minimal padding, but it's rare for a LO to complain about it. 35lb RF limit with a taller shell! 40lb FF limit however most kids outgrow it in torso height at about 3-3.5yo due to the low 15in top slots.. It just doesn’t offer enough torso grow room to get most kids to 40lbs. Readily available at Wal*mart/ Target/ K-mart. Usually requires 1-3 chunks of pool noodle to get a good recline.
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Safety First Uptown: 35rf/ 40ff/ tall 16.5in top slots so this is the ONLY RF/ff 40lb limit convertible that will likely get a kid to 40lbs. EPP foam, velvety plush cover, super cushy. Very easy to use & install. 8 yr lifespan! (Most have a 6 yr) Around $100 at albeebaby.com, Target.com, and in store at Wal*mart.
Here's an awesome thread comparing the Scenera & Uptown on carseat.org. Side by side pictures.
http://www.car-seat.org/showthread.php?t=10324
The Graco Comfortsport, Evenflo seats, and the Alpha Omega 3in 1 types are very small seats that are usually outgrown the same time as the 40$ Scenera. You don’t get much for your money. While they are safe there are just other options that offer more for the same cost.
Edit to add: and while your dd might not seem to need a higher weight limit seat another feature that these seats have is taller top harness slots so they also accomodate taller/longer torso'd kids much better too. My ds was just shy of three and only 27lbs when he outgrew his 40lb limit convertibles!
hth!!