My friend Sandy has taken the training to be a local "car seat safety inspector" in our state of Indiana. What I have learned from her is that we basically need to keep on using these seats... for a LONG time!
Sandy has learned that safety belts are made for ADULTS, at least 80 pounds and over. She also asserts that using a lap-belt only, or using a lap-belt and tucking the shoulder-belt behind the child, is as dangerous as using no restraint.
Every child needs a carseat/booster seat until EIGHTY pounds. I believe this is now even the law in some states.
Get a new booster seat that is rated up to 80 pounds. A Century Breverra is a good one. If your child is not yet 40 pounds you can select one that comes with special straps for smaller children. Once they hit 40 pounds (I think -- read the literature with the seat) they can use the booster with your vehicle's lap/shoulder belt. The nice booster seats have a little guide that properly positions the shoulder belt. This belt should cross the child over their shoulder, not their neck or chin. The seats also have a lap guide so the lap belt protects the child across the pelvis and doesn't ride up onto their tummies.
Sandy has explained to me that what these seats do is position the child properly inside the straps so that if there is any sudden impact, they contact the straps against strong bones, and not vulnerable organs. Many of the seats also have a headrest to protect against whiplash.
Sorry if I sound like an old nag here, but I know so many parents who say "no biggie after 40 pounds" and I know how devastated they would be in the event of a tragedy. Having a friend trained in this area has really taught me a lot!
Hope this helps!
Colleen