That's my point. Philosophically speaking, the statements are contradictory: both vehicles share many of the same parts, and both vehicles do not share many of the same parts, yet in actuality both of these statements are true. Both vehicles are similar in design and purpose, yet both vehicles are not similar in design and purpose. These statements directly contradict each other, yet in actuality they are both true, depending upon how we define their design and purpose.
The statements are contradictory, but only one of them can be true. How can you possibly demonstrate that a car has most of the same parts as a truck and does not have most of the same parts of a truck? There is nothing true about what you are saying if you claim both.
Gravity is an observable phenomena. The theory of gravity is an attempt at quantifying and predicting that phenomena. Evolution is also a phenomena, but a far more difficult one to observe. And it's also far more difficult to quantify or predict. And therefor more likely to produce apparent contradictions.
Then there are theories that can't be observed, and that involve phenomena that can't be quantified or predicted, like the events of a murder that happened months ago. Or the likelihood of some deadly event happening in the future. Few mysteries are as easily observed and quantified as gravity. And yet we still have no idea what gravity actually is.
Evolution isn't that much more difficult to predict or quantify than gravity. If you would like to point out these supposed contradictions in evolution, feel free and I'll do my best to answer them.
In your second paragraph, you are jumbling the colloquial usage of "theory" with the scientific usage. Gravity and evolution have been repeatedly tested and fulfilled predictions and we know they are true because there have been no contradictions to the tests and predictions. There is a lot of evidence backing both up.
But for your murder scenario, what you really mean is "hypothesis" or "educated guess". The same scientific usage cannot be applied because the educated guess has not undergone numerous and repeated tests. There have been no predictions made, either.
If a hypothesis is contradicted by evidence, we discard it. If a theory is contradicted by evidence, we either discard it, or come up with an explanation for the anomaly.
And yes. We CAN predict future events based on predictions of current scientific theories and laws. Meaning if there are any contradictions, the theory is no longer reliable and we no longer accept it as valid. We can tell when a comet will collide with a planet. We can predict the number of metres the ocean would rise if the Earth gets hot enough for the ice caps to melt. We are getting closer to being able to predict when earthquakes are about to occur.
And yes. We CAN predict past events based on predictions of current scientific theories and laws. We recreate the scenario and can determine what exactly happened, or at the very least, what
probably happened. We can measure the speed at which land masses move, interpolate that back millions of years and get an idea of what Earth looked like then. We can examine fossils and see what kind of creatures roamed the Earth. Look at their claws, bones, teeth, and determine if they were carnivorous or herbivores.
If in your murder scene, the victim was shot with a .22, we can look at the original position of the body, where the bullet entered and from what angle and determine whereabouts the murderer stood and from what distance as he fired the shot. We then examine that area for evidence. Hair samples, fingerprints, shoe-prints, etc. We match them to suspects and can safely conclude they are the murderer. We can tell exactly what happened and with accuracy.
The point in this is, if there is any contradiction to be found, that negates the reliability of the claim/hypothesis/theory. So what you are saying is indeed incorrect.