Some critics will say that the federal government's decision to buy a lot of Chevrolet Volt plug-in hybrids for the Pentagon will artificially inflate sales figures for the extended-range plug-in. Others will point out that there is another program being promoted to move Volts, a dealer-incentive program initiated by Chevy parent General Motors.

GM has been instituting a so-called stair-step incentive program that boosts per-vehicle cash payouts to dealers that exceed targeted goals for selling the model, Automotive News reports.

Take the story of dealer Ourisman Rockmont Chevrolet near Washington, D.C., which generated almost $100,000 in incentive cash from GM this summer. The Maryland dealer sold 33 Volts in July and August, with demand spurred in part by the dealer's own $5,000 sticker-price mark-down, $9,500 in state and federal tax credits and a cheap leasing program.

Such incentives were among the factors that pushed the Volt to monthly sales record numbers last month. That's when GM sold 2,831 Volts, more than nine times as many as August 2011.

Earlier this month, reports came out saying that the U.S. Department of Defense will buy as many as 1,500 alternative-power vehicles, including many Volts, in order to cut pollution and refueling costs. Right on cue, some conservatives are decrying what they say is the continued taxpayer subsidy of a less-than-successful vehicle.