THE QUESTION in the ears of America is: Who is REO Speedwagon?

The answer is stranger than publicity. The band came out of the midwest 10 years ago and spent a decade on a grueling cross-country concert circuit, releasing 10 albums, none of which ever went higher than No. 33 on the charts despite belligerent titles like "This Time We Mean It."

But this year, familiarity bred success. The rock's quintet's new album, "Hi InFidelity" has held the No.-1 position in the album charts for six weeks, turning triple platinum somersaults in a depressed record market. And their hit single, "Keep on Loving You," also climbed to number one.

"Now we worry about when it's going to fall," laughs lead singer and songwriter Kevin Cronin, 29. "If it goes to No. 2, are we washed up? Hey, No. 2's not too bad!"

A leading indicator of the group's success, lead guitarist Gary Richrath confessed in a quiet dressing room after REO's recent Capital Centre concert that he'd lost a bet with his girlfriend and had to buy her a special gift. What kind of gift? "A Cadillac, either a Seville or an El Dorado," Richrath says with the little-kid-in-the-toy-store exuberance typical of rock's nouvea riche. The group, named after a high-speed truck designed by Ransom Eli Olds, has shifted into overdrive.

If "REO who?" is still a fair question for many Americans, it's not for want of hard work by the group. In the last 10 years, REO has performed for almost 4 million paying customers on the concert circuit, setting a slew of sales records around the country (including 50,000 tickets in four hours in Chicago). One reason that success was slow in coming: an identity crisis in which REO was perceived as a stereotypical hard rock/heavy metal band. Over the last few years, REO started to temper that metal with melodic ballads, and that has allowed them to develop what one industry veteran calls a "mass appeal sound." And what appeals to the masses sells to the masses.

"It's no surprise that it took 10 years for our success to happen," says the 31-year-old Richrath, who often has trouble getting in a word on the speedy Cronin. "It's not an easy thing to do. We do play fast and loud. That gets people up on their feet and jumping around, having a good time. It lets them get their energies and hostilities out. But we get just as good a reaction for the slow songs. They can be just as powerful. We're a powerful band."

To get that power, Cronin and Co. have had to overcome a series of barriers: a record company not quite sure how to sell them, radio stations not particularly anxious to play their songs, and critics quite anxious to dismiss them as unimaginative pop rockers unworthy of Elvis Costello accolades.

"What we were is what we are," Cronin sputters, "a rock-and-roll band with good songs and melodies." Cronin has just reluctantly abandoned the Capital Centre stage after peddling himself for 90 minutes -- and a gallon's worth of gatorade -- to 20,000 screaming fans.

The audience seemed evenly divided between male and female, a factor not lost on the group. REO is aware that most hardrock bands usually draw an audience that is three-fourths male. "We have something for each," Cronin points out. "On songs like 'Ridin' the Storm Out,' the guys can jump up and down, while the girls can get off on 'Time for Me to Fly' or 'Loving You.' "Most of the crowd was under 20, perhaps the reason that Cronin admits, "I lied tonight, I don't even know why," he laughs. "I started off a song with 'In my 25 years . . . ' If I was 25, it would have meant our first album came out when I was 14." He reconsiders a bit, "I do feel . . . 18."

Back when he was 18, in the late '60s, Richrath and Cronin were working in separate bands on the Champaign, III, bar circuit and booking agencies gave them Top-40 song lists. "You have to be able to play at least 15 songs on the list or you couldn't get work," Richrath recalls. "It was tough to play original music, people wanted to hear a jukebox on stage."

Both found a home shortly after REO was founded by drummer Alan Gratzer, now 32, and keyboard player Neal Doughty, 34, who came up with the name while studying transportation history at the University of Illinois at Champaign. Bassist Bruce Hall, 27, and Richrath joined soon after. Richrath's mother served as unnofficial publicist for the group, haunting Chicago record stores to make sure her son's product was always in plentiful supply, even when there was no matching demand. For a number of years, the band was managed by another UI-C drop-out, Irv Azoff, who after parting with REO went to manage the Eagles, Box Scaggs and Steely Dan and produced both the film and record versions of "Urban Cowboy."

Like the other band members, Cronin grew up in Chicago, playing many of the same stops on the frathouse and singles-bar circuit. He was with the band on its first two albums in 1970 and '71, left after personality conflicts with Richrath to pursue (unsuccessfully) a solo career, and returned in 1977 -- in time for the group's first live album and first solid commercial success, a double album prophetically titled "You Get What You Play For."

Bob Seger and Peter Frampton have already used the same logic to achieve their first major sales success: If an act is terrific on the road, but the studio efforts don't sell, then the magic must lie in the live performances. By releasing a live double album, reinforcing the concert experience, these bands could give their most loyal audiences the equivalent of a 90-minute concert. The process worked again for REO, and their album breakthrough in 1977 has become a model for bands who are successful on the road but unable to break the constrictions of radio and airplay.

And REO was constricted by many radio programmers. In rock's standard Catch 22, REO couldn't get airplay until they were famous and they couldn't get famous until they had airplay. After winning a fight with their record company, Epic, to get the double album, Cronin says, the band found that "it was only getting play on four stations in the country. Since the record company didn't know what to do with it, we got behind ourselves. We sat in an office for a week with a list of program directors from all over and started calling them up person to person. We said it was our last chance and to please give it a chance, listen to our music. They flipped out at the idea."

Eventually, the ploy paid off. So did their decision to start producing their own records and "it's been uphill ever since. We worked our a--s off for this," Cronin growls, leaning forward in the dressing-room chair. "We deserve it."

Their sudden success "doesn't sink in," Richrath admits. "We go on tour and every single concert is sold out. People are listening to the old stuff [six of the group's albums are back on the charts, rivaled only by the Beatles post-Lennon resurgence]. It's a lot more fun."

Having played the Midwest for all it was worth, REO decided to move to Los Angeles in 1977; along with the live album, that became a turning point in their career. But they weren't ready to give up the rewards of the road. "We like playing, that's the basic thing," Cronin says. "We don't like hiding in our houses. Actually, it's easier being on the road than being at home."

Home is where the hurt is, at least according to People Magazine, which featured the group in last week's issue under the headline "Now It's Cheat to the Beat." Cronin is particularly perturbed at People for concentrating on how he learned about a premaritial affair his wife had had, and how it became an inspiration for "Keep on Loving You." "They took a true story and blew it all out of proportion," says the distrubed singer. "Anyway, it was not one of the happier things in my life."

Cronin is also a bit upset with Rolling Stone, which ran a huge picture of him and a "2-inch square picture of the band. We're a group, " he grumbles. Cronin is not yet entirely comfortable with the toll booths in the fast lane.

Right now, though, the band is in the midst of a long nationwide tour in which, for the first time, an album is supporting them rather than the other way around. Instead of becoming complacent, they have chosen to look over their shoulder -- and just below them on the sales charts -- at their old friends from Cleveland Styx. That group's "Paradise Theater" is firmly entrenched in the No. 2 spot on the album charts, and Styx (voted the most popular band in America in a recent Gallup poll) will follow REO into the Capital Centre in early April.

"We go back a long way," Cronin chortles. "We played together for years in different combinations: Us headlining in one town, them in the next. And they've had a No. 1 single, but they've never had a No. 1 album -- and our album is what 's keeping them from having it . . . and it's driving them crazy . . . and I like it."

"It's a healthy competition," he adds quickly. "We're used to this battle of the bands, it's just that now it's on a nationwide level . . . and we're winning." He sits back, satisfied. "It's the American dream come true, the work ethic. You work hard, you stick togetehr, and sooner or later . . . you're successful. It's fun to finally be winning."