Remember, when pulling the best car out of a PSO, make sure to use the Best Car button. As you imply, the PSO does not guarantee that each generation will contain something as good or even close to the best of the previous generations. The best car button will export the best car found over all generations, not just the current one. And with the evaluation logic, any ties in score should be broken by which one has the lower completion time, so it should return the fastest. (If it isn't then there is a bug)
It can be frustrating to unable to tell whether the PSO is actually finding improvements to time at a glance; a request to add a time to completion part to the history graph has already been made.