Investors have been buoyed by strong sales of GM's big trucks - the 2014 version of the Chevrolet Silverado shown here.

Barely four years after emerging from a bankruptcy some thought it might not survive, General Motors is once again being seen as a barometer of the American economy, the maker rejoining the S&P 500, a key stock market index, after the New York Stock Exchange wraps up Thursday trading.

GM will replace H.J. Heinz Co. on the S&P 500, according to the S&P Dow Jones Indices. It comes as the latest in a series of endorsements for the Detroit maker, including recent debt ratings upgrades, and follows a series of strong earnings reports and U.S. sales gains.

The move is expected to help further boost the share price of GM which has already experienced a more than 25% gain since early April. But it’s by no means alone.  Cross-town rival Ford Motor Co. has jumped by an even greater margin in recent months, as has Fiat S.p.A., the Italian automaker that now owns a controlling stake in Detroit-based Chrysler Group LLC.

GM isn’t the only maker regaining a seat on the S&P 500. It will be accompanied by AIG, the giant financial firm that also struggled to stay afloat during the depths of the recent recession. AIG will replace Baker Hughes, Inc. on the closely watched index.

It took $182 billion of a highly controversial bailout fund known as TARP to keep AIG afloat.  By comparison, GM received a relatively modest $50 billion from the U.S. Treasury. The automaker’s bailout is expected to result in a loss to taxpayers that could exceed $10 billion depending upon just how high the government’s remaining shares climb in value.

The White House has said it expects to sell off its remaining GM holdings by April of 2014.

Just how high that stock could climb remains to be seen. The maker returned to public trading in November 2010, its shares priced at $33 apiece.  After a brief climb, however, the stock took a tumble – along with most other automotive shares.

(How high is up for GM Stock? Click Herefor that story.)

But GM finally topped the 2010 IPO price last month and has continued gaining momentum. It opened today’s trading at $35.01, a 1.7% gain over Monday’s closing price and a significant jump last July when it bottomed out at just $18.85 a share.

Ford opened for the day at $16.01, a nearly 1% gain.  While it seemed to sputter a bit after the morning bell, the second-largest domestic maker has nonetheless posted a marked rebound from an early-April low of just $12.44 a share.

Fiat stock, meanwhile, has nearly doubled from its 52-week low of $4.19 a share, jumping to $8.22 on Tuesday.

GM’s rebound appears to reflect a number of positive factors including unexpectedly strong May sales, particularly for high-profit truck models such as the Chevrolet Silverado and GMC Sierra pickups, as well as the newest offerings from the Cadillac division, which posted its best sales in a quarter century.

(Auto sales surge sharply for May. Click Herefor the story.)

GM first-quarter earnings also encouraged investors despite a 14% drop due to some special, one-time write-offs and continuing problems in Europe. But the maker’s latest European turnaround plan, which includes a German plant closing, appears to be getting a positive reception.

(For more on GM’s latest earnings, Click Here.)