But figuring mileage is more complicated. Operated by Clean Energy, the natural-gas firm founded by T. Boone Pickens, the pump listed CNG at $2.59, while an adjacent Marathon pump offered diesel for $4.11 per gallon. However, CNG was listed in gasoline-gallon equivalents. It takes approximately 12% more gasoline to get the same power out of diesel, so we effectively filled up with 31 gallons of "gasoline," or about 27 gallons of "diesel," for $80. Filling up with this much diesel would have cost about $111.

The job went quickly until a taxi pulled up next to us. As the driver drew CNG out of the pump, it was as though he flushed a toilet while we were in the shower: Pressure dropped rapidly, and what should have taken about 10 or 15 minutes extended to 25. If you have about $2 million to spend, you could install your own fueling station and trickle-charge your trucks at night, Boyce notes. On CNG investments, most fleet owners look for a three-year payback or less, he adds.

The downturn has put pressure on fleets like Ozinga to squeeze life out of older diesels, but the firm is now in replacement mode. This truck is Ozinga's first new mixer since 2006. Having done the math, it is adding 13 more CNG trucks and a fueling station this year.