Here's an easy, quick way to do it using the "auto" ratio setting. (You can go deeper into this, of course, if you want shorter lower and longer upper gears, etc, but this way's pretty straight forward, I think).
When you're in a race, go to qualifying and pay attention to the gears, especially in the longest stright. Ideally, your car should hit its peak in top gear shortly before you begin breaking after the longest straight. This way, you're making full use of the engine's full range on a given course. So, of course, Monaco will be extremely short, while the Test Course will be very wide.
You can adjust this a bit, of course, given the track. If, say, you wanted to favor acceleration on all SS11's corners, you might shorten the gears so you're bumping the limiter sooner on the long straight.
(You can also make the top gear a bit longer, too, if you want. The risk in doing that is having the revs drop too far when changing from, say, 5th to 6th. If you want to go that route, take a look at the link Talentless posted in the other thread).