Brother finally its good to see someone driven Prosmatec close to 300K.
Last time i checked it was 46L with reserve in both my 2001 EXi Prosmatec and 2005 VTi Prosmatec so it is pretty much the 50L as written in owner's manual.
The real question is the fuel economy which depends greatly on condition of car.
Normally 2005 Civic Prosmatec goes 70-80 km / cng tank Manual goes 100-120km on city / highway.
Petrol average is 7-8 km/L for Prosmatec and 9-10 for Manual. Naturally the VTi is a gas guzzler and drinks fuel like anything sometimes even more than Reborn.