DH in Italy in 2000-2001 had a Lanos (Hyundai) that he had modified to run on LPG. It's not uncommon here.. but you have to find space to fit another 10- or 20-gallon tank, usually in the trunk, or in place of the spare tire. They have special gas stations with a special pump that connects to a standardized extra 'input' in your car. Not something you would really want to DIY unless you know what you are doing.. the LPG is a lot more dangerous and flammable than gasoline. There are prohibitions here about underground parking for LPG cars (not for the risk of the car itself exploding, but the possibility of small leaks collecting over time from various cars.. since the LPG is heavier than air it could apparently collect and 'stagnate' and be susceptible to any random spark).
I personally would only buy a model specifically manufactured to run on both, rather than a retrofit.. DH had some performance problems.. (not a mechanical person so I can't tell you exactly what). Apparently the engine was not entirely happy switching back and forth. Can't tell you about longevity 'cause he sold it, but you do end up running both, as LPG gas stations are fewer and farther between. In theory the LPG should burn cleaner, so the engine should last longer (?) but I have also heard stories of certain impurities building up and ruining the motor. Here a retrofit would cost €1000-1200.
A quick look at the main Italian car mag site lists 49 new bi-fuel models currently sold, mostly Citroen, Renault, Subaru and VW.
Certain cities here offer incentives for those who convert their cars to LPG, and LPG cars can get around the traffic limitations imposed to reduce smog. Something they should think about for L.A.!!
Here avg. gasoline price is €1.28/liter.. Diesel is €1.21 and LPG is €.67. That's $5.86/gal., $5.56/gal., and $3.06/gal., respectively.* Diesel also used to cost half the price of gasoline, but then people started buying diesel cars and whoops! it "mysteriously" went up. Probably will happen here with LPG as well. It's on the rise (the dates are written "backwards" here; these are prices from July 1, 2005 and January 10, 2006):
Media benzina 01/07/05 (senza servizio): 1,238
Media gasolio 01/07/05 (senza servizio): 1,124
Media GPL 01/07/05: 0,555
Media benzina 10/01/06 (senza servizio): 1,247 (+ 0.7%)
Media gasolio 10/01/06 (senza servizio): 1,149 (+ 2.2%)
Media GPL 10/01/06: 0,669 (+ 20.54%)
I'm convinced it will be roughly equal to gasoline w/in a few years.
There's also the possibility here of converting to methane. It's priced by the kilo. (91 cents/kilo). But it seems you need a way bigger tank.. Lighter than air, so no parking issues.
With the good new little diesel engines it appears that the cost isn't so different from LPG, though. Here a random poster cites the cost of going 100km in two Fiat Puntos: €5.40 with LPG; €6 with diesel.
la mia punto fa i 13/l...in 100km servono 7.7l di GPL x 0.7e/l=5.4 euro (per 100km percorsi)
- le punto attuali diesel mtj 1250 fanno almeno i 20km/l....in 100km servono 5l di gasolio x 1.2e/l=6 euro (per 100km)