With inventories of the Hummer H2 and H3 piling up amid high gas prices, two Detroit area Hummer dealerships are getting creative about boosting fuel efficiency. But the move has raised the eyebrows of General Motors Corp. and Hummer brand brass.

Detroit Hummer and Hummer of Novi are trying to woo consumers with what they call the "Mileage Maximizer," a mechanical modification they claim helps the hulking vehicles get 25 miles per gallon in highway driving. By comparison, an unaltered H3 gets 19 to 20 miles per gallon on the highway, according to Hummer spokesman Nick Richards.

The improvements come at a price, though. GM and Hummer officials haven't endorsed the adjustments, and say they may not honor the warranties on the altered vehicles.

Even so, the promotion has helped ignite H2 and H3 sales at the two Detroit area locations, says Gary Krupa, general manager of Hummer of Novi. The dealerships are charging customers $189.95 for the "Mileage Maximizer." Mr. Krupa says they sold about a dozen modified Hummers in the first three weeks of the promotion. The chain is now considering modifying vehicles at other Detroit-area Chrysler, Jeep and Dodge stores it owns "within the next month or so," according to Russ Reimer, the service director who runs the service shops at both Hummer dealerships.

The modification uses a device manufactured by Air Synergy Labs Inc., one of hundreds of aftermarket parts companies across the country that are using homegrown methods to try to boost fuel efficiency. Spencer Robley, chairman and president of the Las Vegas-based company says his company's product -- which it calls a "Vortex Valve" -- can help drivers increase fuel efficiency as much as 30%, though he concedes there's no official verification of that claim. "Federal, state, local [government agencies], nobody will certify anything that has to do with us," Mr. Robley says. "Nobody wants to hang their hat on it and certify anything that has to do with mileage." The company says it has sold 120,000 valves since they launched the product in 1998; the Detroit area stores are the first car dealers to feature it.

According to Synergy Labs, the valve works by rechanneling and increasing the air that goes into a vehicle's intake manifold, allowing more oxygen to be present during engine combustion. As a result, the company claims, fuel is burned more efficiently. Mr. Robley says the technology can be used in any vehicle's engine and with any kind of fuel, including petroleum, hydrogen, and ethanol.

Hummer's Mr. Richards says that while GM hasn't had a chance to examine the vehicles, the company generally advises consumers and dealers against making tweaks. "We spend years optimizing everything for performance, ride and handling, overall vehicle efficiency, durability and long-term reliability and to meet current and future emission standards," he says. "We've got a lot of very smart engineers who have been doing this for their entire lives. If it were that simple, they would have already discovered it."

John Millett, a spokesman for the U.S. Environmental Protection Agency, which regulates vehicle emissions, says the agency isn't aware of the Hummer promotion in Michigan, but warns that attempts to boost fuel economy can sometimes increase vehicle emissions. "It's safe to say, in general, that in any situation, tampering with the emissions controls devices on any vehicle certified by the EPA is against the law," Mr. Millett says.

Mr. Reimer at the Hummer dealerships contends that "if you are burning the gas better, you are actually creating [fewer] emissions....We've had some customers already call us back and say they have seen an improvement."