I've been researching safety seats for a while because he hit 18 lbs at 4.5 months, and I was like, "Ok...time to figure out what to do next..." The whole car seat issue just fills me with righteous indignation at the way manufacturer's capitalize on parents' legal obligation to meet minimum safety standards by requiring you to buy 3-5 safety seats to get them through the 80 pound mark required by many states.
At best, the median weight for 1 year old boys is 23 pounds, so the "standard" infant car seat, which stops at 22 pounds won't even get you through a year if you have a boy. If you're one of those people who want to buy a travel system, about 75% of them come with 22 lb seats but 35+ lb strollers. The highest weight I've seen on a travel system car seat is 30 lbs.
Next there's the law. Just about every state I know of goes by the NHTSA guidelines for infants which is 22 lbs AND 1 year in a rear-facing seat with a 5-point harness. Many states like Virginia strictly go by the NHTSA height and weight requirements for older children regardless of age: 40 pounds before switching to a booster (age 4 suggested), and 80 lbs AND 4'9" (age 8 suggested) before going to a plain seat belt. I would hate to be a 4'5" 12 year old in Virginia. Georgia is more lenient. They require all children six and under to be in a restraint "appropriate for their weight and height".
The first reason to for these guidelines is physiological. Even though an child is a certain height or weight, they are still at a certain stage of development based on their age. Infants and toddlers have fragile bones. Their skulls aren't fully hardened until around age 3, which is why rear facing car seats are the safest for as long as you can do it as they greatly diminish the force of impact (a physics lesson in and of itself) and a large part of what protects your internal organs during heavy impact is toned and developed musculature. Infants and small children obviously don't have that.
The next reason has to do with physics but I'll forgo the lesson here as well. It is recommended that infants and toddlers under 40 pounds be restrained with a 5-point harness because a properly fitted 5-point harness diffuses the impact across the rib cage rather than across the abdomen and pelvis as a 3-point harness or lap and shoulder belt would. The largest reason for child injuries and fatalities in car accidents is not the actual impact, but injuries caused by an ill-fitting harness or putting too small of a child in a 3-point or adult seat belt. Common injuries include pelvic fractures and abdominal strictures.
Now we have cost versus value versus convenience.
Since child product manufacturers want to rape parents for all they're worth, most people end up needing to upgrade their infant car seats when their child is between 5-9 months unless they got a 30-pound car seat. That means you need a rear-facing car seat that can transition to forward facing in order to get the most "value". This is known as a convertible car seat. You can try to be smart and get a convertible car seat for a newborn since most of them claim to be good starting with 5 pounds, but a car seat that can accommodate a 40+ pound toddler ends up swallowing newborns without lots of extra accessories to ensure proper fit and support. The good news is that you can get a 40-pound convertible car seat for $35-40 (Cosco Scenera Convertible Car Seat). Pay a little more, and you can get the Evenflo Tribute for $60 which accommodates a taller child. For even more ($70-100), you can get something in Evenflo's Titan series which has a maximum weight limit of 50 pounds, but is strangely a slightly shorter car seat than the Tribute. It is supposedly good for children up to 50" tall, but I've read many reviews from parents saying that their children outgrew the Titan car seats in the 30 something inch range. For costing 2x or more than the least expensive convertible car seat, I think it better last as long as it's supposed to. You can pay up to $150 for a low-mid price range convertible car seat, but the max weight is 65 pounds on these and they are HUGE--not great if you have a small vehicle or frequently ride with passengers. For the $150-250 price range you can get a convertible or 3-in-1 car seat that turns has a booster mode for children up to 100 pounds. Most of these are huge as well--the bigger the kid, the bigger the seat.
Want to know how much a forward-facing only seat costs? The least expensive toddler seat appropriate for 1 year olds that I've found is the Cosco high back booster, which is forward-facing for 22-40 pounds and a belt-positioning booster seat for 40-80 pounds. It costs $49 even at walmart.com. That will get you through even the strictest state requirements.
If you don't have special needs, the least expensive way to go is to get a cheap convertible car seat, then a forward facing seat when they outgrow it, and a $15-20 backless booster afterward if they need it. If your convertible car seat allows you to go straight to a booster mode, you can get a high back booster for little kids for as low as $30. But this route requires up to 4 separate seats, which isn't convenient and carbon footprint decreasing and all of that. If I wasn't constantly having to spend money elsewhere, I'd spend more upfront and get the most compact convertible car seat with the highest weight capacity I could find rather than having to buy so many additional seats. During my car seat search, my favorite has been the Recaro ProSERIES because they're compact, have a high maximum weight capacity, and are made in the USA as opposed to China where all the other car seats are made. They also seem to be the safest because they have really high standards and put their focus on safety and convenience and ease of use as far as making height adjustments.
If you're curious about which car seat I got, I picked the Evenflo Tribute. I was strapped for cash and had to put aside my preferences and ideals so that my little man would have something ride in. The features are pretty good: high user ratings, two position recline, latch equipped (safest and easiest way to install a car seat, compatible with most cars manufactured from 2002 on), body and head pillows, fold down cup holder, and able to withstand about twice the impact as the minimum federal crash safety standards. I wish it had a higher weight and height limit, but my son should be about a year and a half before he reaches 40 pounds because babies and toddlers slow down a lot with the weight gain from 6 months on. At that point, I'll have more flexibility in choosing the style of car seat , and maybe I can afford the one I like then.
My car seat will come in between 8/12-8/17. I'll tell you all about it when it comes!
I hope this helps some people in their car seat search.