What are the Effects of Extra weight in cars?

The effect of extra weight in a car depends on a lot of varibles. Like how much weight, where is displaced and what kind of car. There's a lot of engineering that goes into that kind of question. Too much weight makes a car handle badly and not brake well. Too little weight does the same thing. Race cars displace their weight carefully. They try to get a perfect balance of handling and performance. The rule of thumb is that the less a car weighs, the better the mileage.