Generally starting ratios are based on experience. If you're a newbie your best bet is to ask one of the locals for a starting ratio until you gain enough experience to make a good guesstimate yourself.
Once you have a ratio you run the car. The simple rule I allways use to get close to the final ratio is this:
- If the car appears to hit top speed before the half way mark on the main straight you are probably undergeared (gear ratio too high). So use a larger pinion to gear up (lower ratio; I know, confusing...
).
- If the car hits top speed after traveling 2/3s of the main straight you are definitely overgeared. Use a smaller pinion to gear down.
Once you manage to get the car to hit top speed between the 1/2 and 2/3 mark on the main straight it is time to fine tune the ratio. Now you look at the infield and determine whether your car is accelerating fast enough out of corners. This requires more experience and is usually not very important for newbies.
The trick is to balance the speed on the straights with the acceleration out of the corners to get the fastest laptimes. On tracks with a long straight but very challenging infield you will probably gear slightly lower to get the acceleration in the infield and as a result hit top speed on the straight at around the 1/2 way mark. If the infield is relative fast (flowing corners) you will want to gear higher as acceleration is not so much of an issue and as a result you will hit top speed at the 2/3 mark.
Hope this helps.