[skyking]

I notice the OP is from 2012, and is long gone now. I suspect it proved to be the low yield burden that many have found it to be.
When it works out, it is usually on pickups that have spare bed space and room for the plumbing, and when the diesel to propane price is closer to 2:1 or better.
Carbon is carbon; Diesel weighs 7.1 pounds per gallon and propane 4.24 pounds per gallon. This is a ratio of 0.597.
Now look at the BTU of each fuel:
diesel = 128700 LHV ( Low Heating Value ), propane = 83500 LHV
This is a ratio of 0.648
Detonation has not been a problem I have heard of when used at the low power settings of cruise.
In a car I would not touch it at all ever, as it involves keeping a pressurized container and potentially leaky plumbing inside your car 24/7/365. I don't mind bringing a bottle home for the barbeque a few times a year, but the other condition sounds a little too risky for me.
If you go the proper route and install a whole certified and vented LPG system, they often use the space from the standard fuel tank for that installation. Not so hot when you want to burn both
I kicked it around when diesel was over $4 and propane under $2, for my 12V Cummins truck. In the end I found it did not work for me, as I don't daily drive a heavy truck when it is not needed. Even my sad Toyota Camry approached 30 MPG on road trips and close to 28 mixed driving. Nothing I could do to the truck was going to touch that.