If running trucks on natural gas is a worthwhile endeavor, then why haven't the auto makers just built them that way? Wait, I know, that wouldn't allow Pickens to suck money from the government teat. It would likely drive the price of natural gas higher for those that use it to heat their homes.
If it is anything like the LP gas used in some farm tractors through the 60s then it is the least efficient fuel option there was.
Less BTU/gallon compared to gasoline or diesel.
Excessive costs of transportation and storage in pressurized tanks made it unfeasible unless very close to a source.
Diesel as a "heavy" fuel is ideal for heavy duty engines as it is slow combusting and therefore much more fuel efficient.
For comparisons of the different fuels in the same model tractor.
http://www.tractordata.com/farm-tractors/000/0/6/64-john-deere-4020-tests.htmlNo doubt in 2009 there have been advances in engine technology and ignition that may offset the differences some but you simply can`t alter the basic characteristic of the fuel.