Hello,
Welcome to the site, hope you enjoy it.
For refference,
next time you want to ask a question about something like this try using the "search" option in the black bar @ top of the page (that bar also has links to "mygtplanet" and other useful stuff

).
Don't take that wrong, you're new here and have never been shown the ropes.
Now about your question.
I'm thinking you are turning off tcs in the options menu and not in the settings menu.
In the options menu there is a little helmet icon with asm, auto, tcs.
The tcs and asm controls of that menu only apply to arcade cars (however, the transmission setting in that menu does apply to sim mode).
If you want to turn off asm and tcs in simulation mode, go to the settings menu found at the begining of races.
Hope that helps.
Welcome to the gtp. 👍
Later.