I don´t agree that taking cars to the 300Mph barrier is simple as getting gears up to Auto 25.
I´m not the best tunner here; I´m not even close to experienced guys like DE, MasterStorm, ViperPilot, Toronado, 2078TM and others, as i have already said here before ^^ But i think i know what i do, as i´m the fastest driver of one car in the leaderboard, and the second fastest Rx-8, losing only to 2078TM, the fastest Driver in the leaderboard, in a difference of 0.03Mph ^^
So, if i could be considered at least a driver who knows a little about Gt4 Max Speed Tuning, i would say:
Tune a car to the best speed will NEVER be simple to be explained with two or three rules. Gearing, Suspension, NoS, Tires, Spoilers, Downforce are extremely important, but as i have learned here, they aren´t everything. The way you shift up, start the NoS, and the way you go into the Test course do EXTREME differences. You need to tune it considering the way you are going to drive, and your way to tuning it´s only good to you driving-style.
Also, i will give you an example. I´ve got 300.68 on my Rx-8. My Auto Gears Settings are not even close to 20.
Hope i have helped you.
Cheers