Fleet owners figuring to slice operating costs by switching to natural gas-fueled trucks are wise to do a lot of figuring first. Unfortunately, for some time to come, doing that will also require some estimating—if not outright guessing.

To forecast how much money will potentially be saved over a new truck’s lifetime by burning natural gas—be it compressed (CNG) or liquefied (LNG)—instead of diesel fuel, a fleet owner would expect to start by gathering projections on what will be the vehicle’s total cost of ownership (TCO).

However, TCO figures for each of the handful of natural gas truck applications typically seen in the U.S are not as easy to come by as those for diesel-fueled trucks of every imaginable description.

To be sure, TCO projections are certainly available from truck makers and leasing companies and firms that track and analyze fleet costs. But it appears no authoritative source yet has a complete handle on TCO for all natural gas trucks.

The biggest part of the problem is the technology featured in natural gas trucks—and thus the trucks themselves— is still so new to the marketplace that even experts find it difficult to project hard and fast numbers on what their ownership costs will turn out to be.

On the other hand, the main argument for converting fleets to natural gas power (environmental sustainability aside) is to cash in on the huge price spread between CNG/LNG and diesel that exists today and that many experts expect to remain in place for the foreseeable future.

It’s difficult not to get excited about projected fuel savings. But having fuel, an undeniably major cost factor, nailed down does not a complete TCO projection make. Truck OEMs and lessors can lay out the current price premiums and the expected maintenance and service costs for new natural gas trucks vs. diesel rigs as well as the cost of fueling infrastructure with reasonable confidence. However, no one can yet speak fully to what residual values will be assigned these vehicles once they ultimately arrive on the secondary market.

“There is a lot of unverified information—speculation—on [operating] natural gas trucks,” advises John Flynn, CEO & president of Fleet Advantage, which provides cost-analytics solutions to fleets including ones operated by Fortune 500 firms. “So, I’d caution anyone to give wide berth to TCO projections.”

According to Scott Perry, Ryder’s vice president of supply management, there are a number of variables to take into account to try and arrive at a TCO projection for a given natural gas truck.

“The traditional model—diesel power—is very deep,” he points out. “On the other hand, we also don’t have a full picture of the costs associated with running EPA ’07 and ’10 engines yet, either. But there is a pool of knowledge to draw on for making assumptions, and TCO can be looked at scientifically.

“Right now, it is Ryder’s view [that payback] will be driven by the number of miles traveled and the fuel consumed,” he continues. “Typically, a fleet that utilizes natural gas for Class 6-8 trucks that run 70,000 mi. or more a year will see a savings. Less than that, the advantage will erode and may fall so far that converting will end up costing the fleet money.”