When you load a tune is when you enter the tire size if I remember correctly.
To find the tire size, there are many options. the best one IMHO is to find from the manufacturer how many revolutions the tire will do in a mile, then divide that by 1609344.
1609344 / Rev per Mile = Tire Circumference in mm
The tire curcumference is what you will enter into the gryphon.
Also if you have a CS or a CTS then you might just put in 35"'s in the setting instead of the number you calculated.
I hope that covers ya, the manual for the gryphon should also explain that a little, might be easier to have the manual right next to you when you put in the tires size
