Solved

XP doubt.

If I buy another experience booster, what effect does it have? Does it multiply, i.e. does it become 4x or does it accumulate?

Thanks

Haven’t tried buying multiple boosts, but I think and hope it just increases the duration; i.e., buying two 2x boosts for 20 races mean 2x boost for next 40 races.
Or else a manager is just 10 boosts away from gaining over 1000 times the base XP :)