Just took a look at the rules regarding tires, and the exact wording is as follows:
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T6.4.1
Vehicles may have two types of tires as follows:
Dry Tires – The tires on the vehicle when it is presented for technical inspection are defined as its
“Dry Tires”. The dry tires may be any size or type. They may be slicks or treaded.
Rain Tires – Rain tires may be any size or type of treaded or grooved tire provided:
1. The tread pattern or grooves were molded in by the tire manufacturer, or were cut by
the tire manufacturer or his appointed agent. Any grooves that have been cut must have
documentary proof that it was done in accordance with these rules.
2. There is a minimum tread depth of 2.4 mms (3/32 inch).
Note: Hand cutting, grooving or modification of the tires by the teams is specifically prohibited.
T6.4.2
Within each tire set, the tire compound or size, or wheel type or size may not be changed after static
judging has begun. Tire warmers are not allowed. No traction enhancers may be applied to the tires
after the static judging has begun
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Is it allowed to have two sets of "dry" or "wet" tires of different compound/make/size? Note that the rule says that the "team must have two types of tires" and that "within each tire set, the tire compound or size, or wheel type or size may not be changed". This could be interpreted as you are allowed to have two sets of dry or wet tires, possibly of different sizes/compounds as long as you don't chance any of these within each set. I know this is probably not the case, but there is no specific wording against it...