I've never used the anti-lag, but the number of kms depends on how hard you drive it - mine is tuned for 1.55bar and i get "around" 350kms per tank. Do you do a lot of freeway/highway driving?

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MY09 Evo X - 1.6bar@3500rpm - got lag?
Barbagallo - 56.03 (S); 1:08.09 (L)
Collie - 50.77
Possibly the most under-driven Evo X in the world??