I've run some very basic tests on variance just to see what the actual effect is, basically adjusting it around and seeing what laptimes come out in qualifying.
From what I can see the scale is 0 to x, whenever the variance is above 0 the laptimes don't drop below the 0 variance times.
The higher the value the less consistent the laptimes are, although I think that one was pretty obvious.
A higher variance seems to increase the base laptime, I expected it would be possible to go faster with higher variance but it seems it's guarenteed the higher the number, ie:
Driver Variance: 0 Mean Laptime: 1:34.132 Deviation: 0.087s
Driver Variance: 250 Mean Laptime: 1:33.737 Deviation: 0.168s
Driver Variance: 500 Mean Laptime: 1:33.323 Deviation: 0.180s
Drive Variance: 1000 Mean Laptime: 1:32.844 Deviation: 0.202s
Thats literally churning out numbers, I'll confess to no knowledge of the inner workings of the game!
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