@FLi_GTi: u missed my point. i am trying to get this guy to RETAIN the petrol engine, even if i love the diesels.
@snasim: yaar, the thing is, economy is not the only thing u should look for. Believe me, my 3C-E is giving me the same economy in local as the CNG gave me, so i didnt benefit ECONOMY wise. what i have now is PEACE OF MIND. NO tuning problems, no underpowered car, no long lines at CNG stations (which always happen in winters). BUT, if u have a new kit, u wont have these problems for atleast a year. and 2.0D suspension in a spectra? that will NOT give u peace of mind. infact, u'll ruin ur cars ride. plus, toying with suspension is dangerous too. i am sure u will end up with excessive understeer, especially when braking too. ur approach WILL work, no doubt, but its a crude approach.
y dont u check with different CNG mechanics to solve ur petrol problem? THIS is the BEST suggestion i can give u. and if u STILL want to put in a diesel, then go for a 2C. thats a cheaper solution then a 3C-E. my car gives me 19/11 with A/C and 22/13 without AC. if u notice, the local consumption is still quiet poor, cuz thats what autos would do in our driving conditions. a Manual should give u around 20/16 with AC and 23/18 without AC (for 3C-E). for a 2C, just subtract 2 from each figure (my rough estimate). but i'd still say, try to retain the petrol, or atleast go for a stronger toyota petrol, such as the 7A-FE or even a 4A-FE.