Most of us find that the online physics in the game slow down your lap times slightly compared to offline practice laps. Best I can tell, tuning your cars you end up with the same results on or offline. But also take notice to the room settings and what Aids you may be using, if the room host has disabled aids, then they will be off of your car without really warning you. As far as the actual tuning, I actually started my custom tuning in a NASCAR at Daytona oval, as each small adjustment was very noticeable at 200 mph. But that only gave me a basic understanding of it. I then chose the car that allready performed fairly well for me, (for me it was '69 Z28) and took it to Deep Forest. Run many laps, identifying your turn entry's so you can duplicate lap times consecutively. Start making adjustments on one setting at a time and notice what it does to your traction, control, and overall lap times. Hope that helps, good luck.
Add: Where you chose to tune your car also makes a difference, as each track has different bankings and curve angles, each adjustment may have different results. I chose Deep Forest because it has such a variety of those traits, a good handling car there can usually be ran on other tracks fairly well.
