Here's a tip mate, take it or leave it.
I usually put the tranny a bit wide at 400 km/h or something top speed for the cars that usually go to 320-350 km/h. I move the first gear all the way to the left like in usual tranny tuning, the second to the left too (but sometimes I don't really put it as fully left), but here's the crucial part. Gears 3 and 4 will be somewhere at the middle and gear 5 should NOT be ratio'ed to the right since you're gonna need a crapload of speed for drafting. In short, gears 3-5 should be at the middle of some sort. But not all should be right in the middle. Just like tranny tuning for circuits, they need to follow the same pattern. So 3 a bit to the left, 4 in the middle, 5 a tad higher than 4. If the car has 6 or 7 gears, then the same pattern will work, but you have to space the gears to fit the number of gears.