I remember to have stuck a BT20 on the rails in monaco 60.. the car hanging with rears on air, spinning them fast made them stretch visibly. But in this case tire expansion is a non issue as it should affect all tires of all cars, and it's not the case that 1 car make his tire +2 cm and the other 0 at around the same speed.
The more internal pressure the more the tire keeps its shape. It's easy to see in dragster videos. For centripetal forces to deform the tire significantly there have to be low internal forces pushing the sides of the tire. As the centripetal force overcomes the air pressure force pushing the sides it starts expanding. As for 2cm being too much, I don't think it's so exaggerated. I'm talking diameter, I should probably have used radius. The weight of the car also affects the diameter of the wheel, not the real diameter but the radius from the axle to the ground. That's the measure that's important for speed calculations. If the weight compresses the tire by 1cm it's like the tire looses 2cm of diameter. The expansion of the tire can overcome that compression due to the weight and even add a bit more to it. The tire material would also affect speed. It would deform more or less depending on how soft/hard it is, and would also affect grip which can have some effect on speed too. These aren't hard facts. I wanted to show that there might be complexities hidden and point one possibility.
On a side note Bernat, it is interesting to add in your explanation, that all that speed that you lose because of this "apparent diameter" end up in heat into the tire, despite you put the same amount of energy into the system.
Bias-ply will expand more than belted radials. Ford GT40 assumed a 3% expansion in their Le Mans testing program.
Sometimes it feels like we're in an actual car club rather than a sim forum. You guys know you (If you own the car, and I'm sure many do) can just go and test the extreme setup values and see what happens? @Bernat concepts are worth highlighting, but if you throw numbers around you should have some solid basis for them.
Interesting theory of increasing the circumference through high tyre pressure in order to ultimately achieve higher speeds. Theoretically and practically comprehensible. However, I doubt that the rFactor 2 code will provide this. I haven't tested it yet. The problem would be that the car then has less grip in corners.
Both directions were discussed above (low pressure leading to more deformation, high pressure giving more effective radius), and I wouldn't rule out the rF2 model doing both (have you seen ttool?) But I suspect the magnitude of any such effects, to the extent they don't just cancel each other out, would be too small for your perceived speed difference. As I said, default setup in that car does 288 no worries. If you're stuck on 281 something is happening, it's not gearing.
Not that big problem with tires. But aerodynamics on the other hand.... The first entry of GT40 in Le Mans was just about to start flying. Maybe it was only luck that they didn't. But rapid tire failure would not have been nice too.
