What????
I think my excel file will be usefull for who want to spare time going on track checking speed.
I've copy and paste this example that i had published a few months ago in GT5 tuning guide thread.
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This is a gearbox i've made for a maxed viper for 1/4 mile .
To know the tire perimeter, i just need to enter all the others datas i have first.
Then i go on the track and i check my speed for a given gear at a given RPM.
After i enter by approximation a tire perimeter to match with the speed i saw on track.
You can build an excel sheet like this with those formulas:
OUT BOX RPM = SHIFT RPM / GEAR RATIO
WHEEL RPM = OUT BOX RPM / FINAL GEAR
KMH = ((TIRE PERIMETER X WHEEL RPM) X 60) / 1000
MPH= KMH * 0,6213