In an interview Sunday afternoon, Mr. Wagoner was adamant that over the next four years, G.M. would make more in past and future cost savings than the money spent on the deal.

"Unequivocally, this was a good return on investment for our shareholders," he said, adding that G.M. saved $1 billion last year alone from cost savings related to the partnership.

He said he saw the deal early on as a way not just to cut costs but also to help G.M. expand its offerings of diesel engines. Nearly half of the cars sold in Europe are now equipped with diesel engines, but the technology is essentially restricted to pickups and commercial trucks in the United States, G.M.'s home market.

In 1999, before the deal was struck, Mr. Wagoner said G.M.'s diesel sales were in the low 20 percent range of its overall sales in Europe. Today, he said they were in line with the rest of the industry.

"That's been half of the big value for us of the deal," he said.

Much of the value, he said, also came from cost-cutting by joining the manufacturing and development of engines and transmissions as well as establishing a joint purchasing operation. The companies will both have intellectual property rights to two kinds of diesel engines used for a variety of vehicles as well as a six-speed transmission. They will also jointly own a Polish plant that produces one of the diesel engines.

The payout is less than a fifth of the more than $10 billion in debt that is carried by Fiat's auto unit and that G.M. risked having to assume.

"It's easy to Monday-morning quarterback the whole deal, but effectively you've paid $2 billion to get out of something that could have cost you a lot more," said Stephen Girsky, an analyst at Morgan Stanley.