Have not been associated personally with CNG.
As far as LP goes: Some hotshot government whizkid convinced several school districts in this area to convert buses to LP several years ago. Result: every single bus either got an engine rebuild, a new engine, or the bus (if old enough) was scrapped. Valves receded through the heads into the pistons in less than one school year. Like to bankrupted several school districts.
Also have personal experience with a factory dual fuel motorhome 454. Mileage on regular gasoline was 9.7 MPG (yes, we kept records). Mileage on LP was less than 4 MPG. Power dropped by about 40 percent. Again, this was a FACTORY dual fuel, Dad bought it new. With a 40 gallon propane tank it was impossible to drive 100 miles!!!
In Missouri, there was also a $110. license annually for using LP. Even when Dad stopped using the LP, he still had to pay the license until the LP tank was removed from the vehicle. Then the vehicle had to be inspected prior to waiving the license. Plus, road tax was added to the cost of the propane.
We got it off before it did any engine damage, but definately not a cost effective unit, even if propane were only 25 percent the cost of gasoline, and the conversion was free!
Jon.
