actually i did it too. trucks in general are more similar to each other then to cars. we are talking about many traits and not just one or two.
Trucks are similar to each other because they are built for hauling big loads. That generally means bigger tires and engines, and a truck bed.
Again, trucks are defined as vehicles to carry heavy loads, and thus all have designs to carry heavy loads. So you are not comparing multiple independent features. The features that trucks have in common over cars can all be determined by four words, "designed for heavy loads". You are simply sorting vehicles by the ability to carry heavy loads.
You can do no such thing for the features that mammals have in common. They have multiple features in common, because heredity has made them that way, not because they needed to be that way.
Dolphins and tuna have very similar environment and functions, but dolphins are placental mammals and tunas are not. Dolphins have the features of placental mammals, such as mammary glands, hair, three bones in their ear, and live birth. They don't have this because they need them and tunas don't. They have these features because of heredity.
Trucks are different. They have the design features necessary for carrying big loads because they were designed to carry big loads.
all placental mammals shared a placenta too. are you saying that this is the product of a common descent too?
Yes, of course, having a placenta and live birth are inherited traits for all placental mammals. It could easily be that a dolphin-like creature would reproduce like tunas. But they don't, because they are placental mammals, and so they reproduce that way.
some cars actually have big wheels too (monster vehicles for instance).
Sure. That does not refute the point that, since trucks are built to carry heavy loads, by definition they tend to have big wheels and big engines, and thus usually cabs that sit high. So the trait to have a high cab is not independent of being designed for carrying heavy loads.
Again, what you are doing is lining vehicles up on one trait, carrying heavy loads. When we classify animals, we are not doing that. In animals there are multiple different layers of division using different characteristics at each division, and producing a nested hierarchy that can readily be seen as correct, and has statistically been shown to be correct. See
Nested Hierarchy: Evidence for Evolution.