Tom Petty teamed up with Jeff Lynne for some of the most irresistibly anthemic music of his career in the late '80s and early '90s — but when the duo reunited for another record in the 21st century, they took a much more mellow approach.

When Petty and Lynne entered the studio in 2005, they had some big hits to live up to. Lynne had been behind the boards for Petty's massively popular Full Moon Fever solo LP as well as his next record with the Heartbreakers, 1991's Into the Great Wide Open — not to mention the two albums they made together as members of the Traveling Wilburys. But Petty wasn't the same guy he'd been back then — and he wasn't the same songwriter either.

After scoring a slew of hits in the first phase of his career, Petty spent much of the '90s making more introspective music, and as the decade closed, he found himself struggling with a series of profound life changes and a period of deep depression. Emerging from the darkness with 1999's Echo, he adopted a less overtly commercial sound — partly due to his state of mind and the passage of time, and partly because scoring further hits was less of a concern than it had ever been.

"I’m a little more into the poetry and the lyrical images than I used to be," Petty told Harp. "I don't want to waste a line, I want to mean something and I want it to be the right line. With this record, I knew that I wanted to have a sound that was cohesive. I didn't want to make a concept album, but I wanted it to fit together sonically. The space is everything in a record. It’s not anthemic at all. I'm real bored with [being] anthemic. I did that and I'm not trying to do it again."

The resulting set of songs, titled Highway Companion, arrived in stores on July 25, 2006 — and if the record's mostly mid-tempo sound might have come as a disappointment for anyone expecting a Full Moon Fever sequel, it was still very much of a piece with Petty's other midlife efforts, and stocked with more gently melodic observations on the human condition from a guy who'd spent his life honing his craft.

"It has a lot to say about time and the passage of time," Petty told Billboard in the weeks before Highway Companion's release. "It's not so much love songs, it's not going to be what anybody expects from me, I'm sure of that. But it's good music, it's really good music."

Sonically, the album offered a bit of a departure from Lynne's previous work with Petty. Although there were still plenty of harmonies and chiming guitars, Highway Companion had a moderately more full-bodied sound — and unlike his prior non-Heartbreakers outings, it was pretty much literally a solo effort, with Petty playing most of the instruments with a little help from Lynne and Mike Campbell. The end result still sounded like a Tom Petty record, but he felt it represented an evolution.

"It's not a real loud record or an all-out rock fest," Petty told USA Today. "It's quieter but not mellow. I wanted to make this for a long time. It's not a record I could have made in the '70s. I wasn't seasoned enough."

In purposely opting for a quieter sound, Petty more or less recused Highway Companion from rock radio, where lead-off single "Saving Grace" only brushed the Top 30 — and subsequent singles "Flirting With Time" and "Big Weekend" failed to chart at all. But despite its lack of a hit song, the album performed respectably on the charts, peaking at No. 4 and ultimately going gold — not that Petty professed to being worried about commercial benchmarks.

"Lately I've been concerned with what I'll leave behind artistically," he mused during his Harp interview. "The biggest priority with the new record now is that I know this is here longer than me and that’s more important than [it] being a hit record. Years ago, you'd have to make sure you had one that was a [hit] single. I don't think that pops up in my mind anymore."

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