In city driving, a 1.5 will get you maybe 60%-80% more economy... judging by your 5.5 km/l fuel economy, you drive very short distances, right? Where do you live? Where do you drive?
A subcompact 1.1 to 1.3 can get up to 14 - 17 km/l in urban driving... but only if you go a certain distance and in certain areas. If you drive less than 2 kilometers per trip, you can still get as low as 8 km/l.
So you'll be sacrificing your car for an extra 3 km/l. So your 500 pesos (at 55 pesos per liter) will get you 72 kilometers instead of 50.
The most cost-effective option is converting the Focus to LPG. A modern SGi kit will cost about 39k + 2.5k for a donut tank. This means you can keep your comfortable "compact" car and you won't have to downgrade to a subcompact like a City or Vios.
Instead of 72 kilometers per 500 pesos, you'll be getting (with LPG's current price of 28 pesos) 98 kilometers per 500 pesos.
So... ano pa? Sacrificing your current car loses you your dual AC, the comfortable ride, the extra noise suppression, etcetera... and you'll still have to pay an extra 100,000 or so on the trade-in. Adding an LPG kit with a donut tank sacrifices a little trunk space, and costs you just 40,000 pesos.
If you're worried about safety, just read Ghosthunter's LPG conversion thread.
I'm converting my 2 liter car this week.
