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Firstly, I set the first gear final drive ratios to 9.274:1 on the Coventry and 7.853:1 on the Lotus, because those are my prefered starting gears. I don't want to have to learn how to start the car differently at each track, so I like to use the same gear.
Next, set the top gear so that the engine redlines briefly at the fastest point(s) on the circuit - then drop the gear back a notch (for reliability and so that you have a little extra for drafting).
Then, because aero drag increases as the square of road speed, set the intermediate gears so that as the gears increase, the drop in revs when changing up decreases (i.e. the drop in revs when changing up from 2nd to 3rd is bigger than the drop in revs when changing up from 4th to 5th). Lack of torque at 150 MPH hurts a lot more than lack of torque at 50 MPH.
Plotting the gear ratios against the gear number on a graph can help when setting the intermediate gears. You can even use regression analyser; for example, the gears for the Lotus at Monza can be calculated using the 'power' formula 'ratio = 7.81 * gear ^ -0.549'. Other cars and circuits use the same formula but with different constants.
Alison Hine: “... Dave Kaemmer suggested that using taller low gears and putting the top gears closer together would be beneficial for lap times. After all, any power in excess of that which overcomes the available traction (i.e. incites wheelspin) is wasted. On the other hand, using high gears that are relatively close together is useful for high speed acceleration because it allows you to stay in 3rd and 4th, near the power peak and taking advantage of greater torque multiplication, for a longer time on long straights.”.
Finally, folks often wonder at what point the advantage of having more gears (and thus spending more time in the optimum part of the torque curve) is outweighted by the loss from changing gear. Answer? In GPL - if you are good at changing gears - use as many gears as you can. Carrol Smith: “Always split the torque as many ways as practical - use the maximum number of gears on any given road racing circuit”.
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