A feud is simmering between Ford and Toyota over Ford's claim that its Fusion gasoline-electric hybrid out this spring will be the most fuel-efficient "midsize car" at 41 miles per gallon in town, 36 on the highway.

Toyota says its higher-mileage Prius hybrid (48/45 mpg) also is midsize — as defined by the U.S. government. U.S. emissions and fuel-economy regulations judge a car's size category by its combined passenger and cargo space.

Fusion has 111.6 cubic feet of combined space; Prius, 110.6. Both are over the federal threshold of 110 for midsize and below the 120 mark for full size.

Fusion is about 800 pounds heavier than Prius, 16 inches longer and 4 inches wider, but the federal regulations don't take those factors into account.

"We are reviewing Ford's mileage claims for Fusion," Toyota Motor Sales spokesman Joe Tetherow says. He notes that the redesigned 2010 Prius, to be unveiled at the Detroit auto show this week, also gets better mileage than the Fusion. The new Prius will be larger than today's, but Toyota hasn't given specifics.

Ford says the more logical comparisons with the Fusion hybrid are Toyota's Camry hybrid, 33/34 mpg, and Nissan's midsize Altima sedan, rated 35/33.

"We've been pretty clear, probably annoyingly clear, to Toyota that we're comparing Fusion to Camry," Ford spokesman Mark Truby says.

The brewing dust-up is significant because it threatens a key point that Ford wants to use to market the car, as well as to improve the company's environmental image. And it signals that auto industry competition is intensifying, as companies fight for slices of an evaporating sales pie.

New vehicle sales in the U.S. last year fell 18 percent to 13.2 million, according to Autodata. That's the worst since 12.9 million in 1992. This year's sales could collapse to just 10.3 million, according to forecaster IHS Global Insight's Automotive Group.

General Motors and Chrysler have borrowed billions from the federal government just to keep the lights on and doors open. Ford says it might need to do that.

In that brutal context, every sale will be contested. Ford forecasts selling about 25,000 Fusion hybrids a year, including a few nearly identical Mercury Milan hybrids. Toyota sold about 159,000 Priuses last year.

"Fusion (hybrid) is the most fuel-efficient midsize, in the way customers shop for cars," Truby says. "If we do use it in advertising, we'll say most fuel-efficient midsize 'sedan,' " he says. Prius is a hatchback, while Fusion is a conventional sedan with a trunk.

If that doesn't satisfy Toyota, the car company could sue, or complain to the U.S. Federal Trade Commission and let it referee. Usually in such ad disputes, though, the combatants compromise.