Sun Jun 14, 2015 7:48 pm by strop
This is something that has been clarified in the past, but probably could do with a more obvious indication somewhere because you're right, simple searches with the search function will not uncover the answer.
The top speed of the final ratio in the drivetrain is ultimately limited by the redline of the engine. That is to say, the higher your car revs, the higher the top speed your final ratio will yield. Your car's engine has a lower redline than the smaller displacements presumably because the reliability of larger engines (esp. larger stroke) suffers at high rpm.
There's been arguments before about this: if larger engines yield higher power, then why can't we raise the possible top speed? The answer to that would be that the range of speeds available on this simple slider already cover the practical ranges of speeds for all feasible production cars, including cars geared tall for economy. The only way cars with ridiculously high theoretical top speeds can even get close to them is if the engine venting was all shut off to minimise drag, and that's an academic exploration of the game's limits not particularly relevant to the main focus, since the reliability of such engines would be zero.