My dd was 37 inches and 33 pounds shortly after her 2nd birthday, 38 inches and 35 pounds at 2.5, and hit 40 pounds at 3 years, 4 months old. She's consistently been in the 95th+ percentile for height and the 85th+ (usually 90th-95th) for weight since 9 months old. And, at 2, she was pretty-long-torsoed, too. She outgrew most 40 pound weight limit seats by height before weight because of the long torso. When shoulders go over top harness slots, the seat is outgrown by height, regardless of overall height, regardless of weight.
Knowing what seat your dd is riding in now, and how she is fitting it (where her shoulders are in relation to top slots), plus what vehicle you drive (year, make and model), and if you have other rear passengers or carseats, will help figure out what might work best for your situation.
As Chickabiddy mentioned, the cheapest options that harness beyond 40 pounds are the Cosco/Safety 1st Apex (which can sometimes be found for around $80 online at Albee Baby) and the Graco Nautilus. The Apex requires vehicle head support at least up to the tips of the child's ears, and it's a bit wider than the Nautilus, with lower top slots (so less height growing room). Both have 65 pound harnessed weight limits, both convert to belt positioning booster seats (the Nautilus becomes both a highback and backless booster).