In one of our 3 cars, dd, at 4'4" and about 60 lbs. DOES fit without a booster. Not only does she meet the 5 criteria points to be without a booster, with a booster, she sits too high and the belt comes across her upper arm instead of between her shoulder and neck as it should.
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So, it depends on the child and on the car. I am absolutely the biggest safety freak, so I would never do anything that I felt was not 100% in dd's safest interests. In her case, using a booster *in this one vehicle* was not as safe as using none. In the other two cars she uses a backed booster in one and backless booster in the other because they fit the best for her in those vehicles. I use what keeps her the safest in each vehicle. I, too, was worried about the whole 4'9" and 80lbs. but the fact of the matter is that it's a guideline and a good one at that... but it's not a hard and fast rule. If the child is safer without a booster, then having a booster is not only not helping... it's dangerous.