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Forza Tune 6
https://play.google.com/store/apps/details?id=com.flamefrontstudios.forzatune6.ft6&hl=en
This is the app i'm using. To do the gearing, it asks for max torque output, and the max redline value. They said to look at this graph and look at the highest point the torque line is at n put in calculator. It has a nice help section explaining things, but i cant seem to find any info as to if this chart readout is a bug? Or if it's suppose to be separated by 5 hash marks like that, or if it should be 4 instead.
And as for the gears, it has 6, lol. And as long as you put racing gearbox on it, u can tune your gear ratios.
BTW that Forza Tune 6 app i've had better luck with my tunes using that, than the free online one.
the one for forza 6 on android works well for horizon 2 & 3 and forza 4 5 6 7 https://play.google.com/store/apps/details?id=com.flamefrontstudios.forzatune6.ft6