On Wednesday, General Motors will unveil a new Hummer that will be smaller, cheaper and less gas-hungry than its predecessors.

The new junior member of the Hummer family, to be called H3, is seen as critical to the survival of a brand that has been treading water this year, as the novelty of the two-year-old Hummer H2 has faded. The H3 is to hit showrooms next spring, a date that could not come soon enough for G.M.'s 167 Hummer dealers, who are facing a sales slump at the same time they are spending millions of dollars to fulfill G.M.'s requirement that Hummers be sold in huge glass and steel Quonset huts.

"The H3 makes that viable," said Jim Lynch, a Hummer dealer in the St. Louis area who is in the middle of building one of the new showrooms. He called the H3 an "extremely important" vehicle that gave Hummer dealers "the volume to really be a standalone franchise."

At a glance, H3 looks a lot like the Hummer H2, which has become an avatar of American swagger or sinfulness, depending upon whom you ask. But there are significant differences. The H3 is about 17 inches shorter than the H2 bumper to bumper, and about 6 inches less wide and tall. The H3 is expected to approach 20 miles per gallon in highway driving and get about 16 miles per gallon in the city, a G.M. official said. That is better than the roughly 12 miles per gallon that the H2 gets. But it is below the 22 miles per gallon on the highway and the 19 miles per gallon in the city for the average midsize sport utility vehicle, according to the Environmental Protection Agency.

Not that Hummer is going after the eco-conscious.

"I don't want you to get the idea we're releasing the H3 to get to those people who think the H2 is not fuel efficient," said Susan Docherty, Hummer's new general manager. Rather, H3's lower price will bring the vehicle to a vastly larger market. The sport utility vehicle is expected to start from $30,000 to $40,000, compared with the H2, which starts at just under $50,000, and the H1, which starts around $100,000. G.M. envisions selling more H3 S.U.V.'s than the combined volume of both of its predecessors and hopes for sales of more than 40,000 Hummers next year, up from about 20,000 this year.