So I have finally decided to try out tuning and I can say it is pretty fun!
I have a few questions though.
What upgrades such as tyres , differential and flywheel be fitted to a drift car?
What in the tuning menu makes a drift car, drift seamlessly?
What is important for drag tunes?
Lastly, I have seen putting a race transmission decreases performance compared to a sports trans in some cars, why is this?
Well, my quick two cents, even though I don’t consider myself a great tuner by any means, but I have tinkered with tuning for a while in this game series. 
Drift Car
Diff - pretty much a must, you will have more control of the wheels locking in.
Flywheel - in lower HP cars it can help, but to me it’s mainly a PI filler.
Tires - in my opinion it depends on the HP, if you have enough to break the rears whenever you want, which most drift cars do unless your limiting yourself to a certain PI, then it depends on your personal taste as to how much grip you want and are willing to do without.
Tuning, drift seamlessly - well, pretty much ALL of the above in the menu. it takes the right combination of all the settings to get the car where you want. I believe the most common rule of drifting is loose in the front and tight in the rear. But it’s going to take some time along with trial and error to get the cars where you want.
Drag
Sorry, I’m not much help here, but a tall gear for launch and enough softness in the rear suspension to squat and grab.
Race tran
Some cars with low HP to weight ratio will give a lower accel number when you equip this. I think it’s because the race trans makes a taller 1st gear and the game’s PI calculator equates that to a lower value in that section.
Add me to your friend’s list if you ever want any help. Maybe I can help out a bit.
GT: CFunk2001
xDeaDxZeppLiNx has some drag tunes up and has a thread were he describes how he sets up a drag tune but I can’t find it at the moment. If you ask him, I’m pretty sure he will share his thoughts.
Way too much to explain in text. Learn what these parts do in real life and you will know what you need soon after. Trust me It works. I didn’t know anything about cars before I started tuneing. Now I know why lancia deltas are in the game.
I have no idea how to tune a car for drifting, I know there are a few settings you can use to help the car break loose, but if you GIT GUD at drifting, all you’ll need is a powerful enough car with a good chassis. Basically a Nissan Silvia, any BMW, Alfa Giulia, etc.
I like drifting with classic muscle cars, too.