hehe...
You just made a timing mistake at changing the gears
But the best way to figure out, that the gearbox is real in TM1 is: to accelerate in island and drive small turns (atbest with analog input). Compare your driving to another (better) ghost and think about the timing, when you should press left/right in relation to the engine-sound you hear.
Just experiment around a bit and you'll notice, there really is a gearbox.
Another example is StarIsland A2. When you just try to noslide there in the tunnel, you are slower, because you loose too much speed in the right-turn, so that the car switches the gear later. You need to interrupt shortly, so that the car is faster and can switch the gear (but it requires some skill to figure it out)
I hope you understood what I mean
