if you get dirt on your wheels how do you get it off, can you wheel spin and itll go away faster or make gradual turns up the track to clean the tyres? does it come off the tyres at a fixed rate, how do you get your tyres clean? or just leave it and keep in mind they are dirty?

Dirt leaves the tyres when the tyre is loaded.
For example: You drive Blackwood GP and hit the dirt somewhere in the first chicane, all 4 wheels are dirty. You drive the long straight and at the end of it your tyres are still dirty.
You pick up some more dirt at the end of the straight because you have a bad day and begin the twisty section of Blackwood where suddenly the tyres become cleaner much faster because due to the the one-sided load in turns the dirt comes off the tyres.

But in the game I just drive. I don't take any special actions to clean my tyres.

Vain

just take it easy for a while till they are clean then.

Be aware which tyres are dirty and think about the next corner coming up. If you've got the right hand tyres dirty and you've got a right hander coming up, then you don't need to change your speed much at all but if its a left hander you'll need to go easy. Soon as you are thru the corner the tyres should be back to full grip pretty much.

Also remember some dirt seems more slippery than others. The sand at Fern bay seems to destroy all your grip where as a little trip on the grassy bits has less of an effect.

does it affect braking in a straight line at all? would i lock up at a brake point where i would normally be ok?

Quote from richy :

does it affect braking in a straight line at all? would i lock up at a brake point where i would normally be ok?

it affects the overall grip the tyre has, so yes, it also affects braking in a straight line...

so if i had like 2 wheels dirty on one side of the car, the car would skid to one side? or would all 4 wheels lock up?

need to start watching that tyre monitor more

1. Depends on wether you use a locked diff or not. Locked diffs make the complete axis lock when one wheel locks, other diffs make it possible that one wheel locks, but the other still grips.
2. Only use the monitor when you want information from it. For example take a glimpse when you want to check tyre wear in a endurance race every 5 laps or so. But don't keep the monitor open all the while, it will distract you.
Let your feel guide you when you picked up tyre.

Vain

thats why i didnt notice it in the XFR then!! good to know!!

the dirt effect is way too strong. nothing more to say about, as you all probably know that

Wether the effect of dirt is too strong depends on the type of tyre. Slicks don't seem to suffer from dirt nearly as much as the road supers, though I'd imagin it to be the other way around.

Vain

I agree - IRL dirty slicks have very little grip but road tyres aren't quite as bad. Gotta say tho - IRL if you put the two outside wheels on some damp grass on the approach to a corner and try and turn in you will end up facing the wrong way. You have almost no grip at all - even on a trackday tyre like a yoko AO48R. I know this because I put a couple of wheels on the grass through Hall Bends at Cadwell Park and when I turned in to the hairpin the car spun in the blink of an eye!