On the topic of tire sizes.
You have to be very careful when you true down tires and the adjustments you make on your car due to that.
Tire size: This CAN(?!) depend on track and conditions. On low to medium grip tracks it's USUALLY better to run slightly larger tires to get what Zoom mentioned, some (!) tire sidewall roll as this will help with grip. On medium to high grip tracks. Smaller diameter tires is the way to go. Due again to tire flex, or the lack of it. It will make the car feel stiffer and more responive.
Car set up: I don't like the term "Ride Hight". Or maybe it's the way it's used.
Yes, ride hight, as used generally, is the hight that the chassis is from the track ! Can I use the term "Ground Clearence" for this post?
This ground clearence should only be altered by trueing down the tires !! This will then not alter any set up attained on the car. And any alteration in car performance or handling is the result of the tires and the lower COG because of the smaller tires!
"Ride Hight" just for the sake of this post (trying to save anymore confusion to set in here..
) is the relationship of the chassis to the axle (be it front or rear) when the car is on its wheels.
So .... altering tire diameter has NO effect on ride hight ....only on the ground clearence. Altering your shock collars has the effect of altering both ride hight AND ground clearence ! And you are then into a whole different set up on your car. Because you are not only altering the distance betwean the ground and the chassis, but also the role centres, droop settings and camber.
Even if you go ahead and check and adjust all these back to your known set up, you are still left with an altered car due to the different role centres and maybe also camber gain
Just from experience, I have found on low to medium grip tracks starting out at 62 or 63mm diameter tires seams to work.
Medium to high grip............... around 58 - 60mm and on high grip tracks, 56 - 58 mm diameter.
Just to make things clear. Using the above tire sizes as an example.
On low grip track with 62mm tires all round you have set your ground clearence to 7mm all round (Just for example)
On a medium grip track that same car on 60mm tires would then have a ground clearence of 6mm all round !
On your high grip track that same car on 58mm tires,would have a ground clearence of 5mm all round !
The ride hight is the same ! Hence your droop and camber settings would not change and your role centre's would also not change. Because you have not altered your shock collars.You have only altered your ground clearence (And lowered slightly your COG.)
This is why setting up your car on set up wheels for ride hight is so important. The ride hight will then be the same every time!
I hope I haven't completely confused everyone. But this comes straight from motorsport, not me......... really.