I have not found much about using LPG along with Diesel to boost MPG but after seeing it on a Chevy 6.5 a friend of mine has been running LPG on and reported 30% improvement in MPG, I decide to give it a try. I have an 06 Jetta TDI 1.9 PD that usually gets about 42-45 MPG hand calc. I added a LPG regulator and small 5 gal LPG tank, piped it into the intake just in front of the turbo inlet. My MFD claims that I have jumped from 48 to over 62 MPG.
I am testing a full tank of Diesel + full tank of LPG to see what the real numbers come out to be but it looks really good so far.
If any one else is interested in trying this it really is simple and not that expensive, maybe $200 if you have to buy everything.
The regulator that I am using came from Impco and is a mod. E. the rest is just hose and shutoff valve and a home made venturi, made from a 1/8 " brass "T", I installed in the inlet pipe to the turbo.
Dieseltim