I developed a spreadsheet to help with tuning your gearbox with less trial-and-error
First step is to calculate the tire diameter. The final result is not 100% accurate, but it is close enough to match what is really happening.
With the Bentley I went onto Daytona Speedway and run every gear to the rev limiter (7000 RPM) and noted the speed I was traveling at. Also ran 1st and 2nd on the pit lane limiter and noted the rpm at 59km/h. I then go the spread sheet and fill in the Final Drive and the Gear Ratios for the respective gears.
All of this calculates the average wheel diameter (and use some savvy).
For the next part you decide the lowest RPM you can dip between shifts, your ideal shift point and your red line. The Torque and HP values are optional and for info only. Not used in calculations.
For the next part you put the gear ratios in (and this is where you are going to play with the numbers a lot.
Based on the adjustments above your numbers below will change accordingly. The RPM number in each column is where it starts after the shift to that gear (check the digital rpm for accuracy, not the analog). The km/h is what your speed will be at the shift point to the next gear and the last column is the speed at the red line. The Bentley has many adjustments for each gear, so it is nice to see how things change as you try to balance the min rpm and max speed for each gear setting. Other cars have limited or no adjustments, so no point in playing with this too much in those cases.
Not to worry, there is a murican sheet for those who want to play in miles per hour....
If you want a copy of the spread sheet PM me and I'll send it to you. If you have questions, ask - what is logical for one might be complicated for another and visa versa.