Choosing Which Ratios to Use
The Hewland website is probably one of the most helpfula nd well written sites around. If you haven't downloaded a manual for your gearbox then it's a good idea to do so and read it as it contains a wealth of knowledge. For example, many use an LD200 (eg Formula Vauxhall Junior). That manual contains several gear charts and the following advice on how to use them. We reproduce it with kind permission of William Hewland.
Using the correct gear ratios is one of the key areas of setting up a racing car. It is far more important to understand for yourself the factors which determine the correct choice than it is to be told by someone else which ones to use. Admittedly, the top teams will usually be using similar ratios but they will have arrived at their choices by decisions based upon their road speeds which is why a beginner may struggle it using the same set.
Firstly ensure that you have a current ratio chart in front of you. This is always available from ourselves [Hewland] by request, You should know the area of useful maximum power in your engine. (For example, it might be between 5,500 and 6,800 r.p.m.)
Then, 2 main considerations should be followed:
1) Ideally, when your final choice of ratios is made. the rev counter will never show outside of this range. (Realistically this is a bit optimistic but you should strive for it anyway). The r.p.m. should be noted on the exit of each corner at the point at which the throttle can be floored. If at this point the r.p.m. is not up to the lower end of the known power band (or higher) and the car cannot be driven faster, then using a lower ratio gear should be considered.
The procedure then is to examine the ratio chart to see which is the ratio to give the increased r.p.m. for the same road speed. i.e. find the line on the chart for the ratio that you are using and follow it to the r.p.m. that you were seeing at the corner in question. You then need to look upwards on the chart to find the point at the beginning of your power band and choose the nearest line/ratio available.
2) The second factor, the r.p.m. drops, should be considered in conjunction with the first and usually obtaining a compromise between the two is the hardest part The point of this exercise, similarly to the first, is to see if the engine will be out of the power curve too much when simply driving up through gears on the flat out parts of the circuit or straights.
This is easy to check. Looking at the ratio chart, follow your 1st gear line up to the point of max r.p.m.: the point at which you will change up. lf you then drop a line straight down the graph until you hit the line of 2nd gear, you will quickly be able to see how many r.p.m. the engine will drop. Simply continue this process from gear to gear. The engine will pull large rev drops from 1st to 2nd more easily than from 4th to 5th for example, so the drops should get smaller as you go up, most importantly not dropping out of the power band in the highest two gears.
Article from Hewland Website