Sprint’s customer satisfaction rating plummeted by more than 8% in the first quarter of 2008, according to the latest American Customer Satisfaction Index, published today.

The ACSI, which is published quarterly by the University of Michigan’s Ross School of Business, finds that Sprint’s wireless service customer satisfaction ratings have fallen to an all-time low score of 56 on a 100-point scale. Sprint’s rating in the ACSI places it significantly behind the ratings of major competitors Verizon, AT&T and T-Mobile, all of which scored 71 or higher in the survey. ACSI founder Claes Fornell says that the drop in rankings is yet another sign of trouble for Sprint, which currently stands as the third-largest wireless carrier in the United States with more than 52 million wireless subscribers.

“Business is unsustainable in a competitive marketplace when customer satisfaction scores are as low as Sprint Nextel’s,” says Fornell.

Overall, the ACSI creates customer satisfaction indexes for 10 economic sectors consisting of more than 200 companies. Each company’s ACSI is based on a sample of at least 250 customer interviews, and the ASCI conducts more than 65,000 customer interviews each year. Sprint’s score of 56 on the ASCI is the lowest reported for any wireless carrier since the ACSI started surveying wireless voice customers in 2004.

Sprint has made improving customer service a key goal of its turnaround plan. Over the past couple of years, the company has been stricken by major customer losses in its wireless services, and the company this week reported that it had shed nearly 1.1 customers in the first quarter of 2008 alone. Sprint was also stung by Qwest’s announcement earlier this month that it had selected Verizon Wireless as its new official wireless carrier, with Sprint’s current agreement with Qwest set to expire in February 2009.

In addition to reporting on customer satisfaction for wireless carriers, the ACSI surveyed customers of all major U.S. wireline providers. In that portion of the survey, AT&T came out on top with a score of 75, followed by Cox Communications at 74. Comcast came in last place in the wireline survey with a score of 69, which is still far ahead of the poor ratings customers gave to Sprint’s wireless service.