Sorry to chime in so late, but here are the specs on my Civic Si -
Tank capacity - 50L
Average KMS - 360km per tank
Price of fuel - $1.50/L (Premium 94 Octane)
Driving habits - aggressive
City/Hwy - 100% / 0%
Engine mods - None (pure stock intake, exhaust)
You guys don't feel so bad now, huh? If I wanted economy, I would have opted for the Hybrid... but 197hp is way more fun! :p