Diminishing returns to increasing car MPG?

The Eco adds apparently $1,600 to the price of the Malibu (with a sales tax of 7 percent that comes up to $1,712). Suppose that people spend about $2,900 on gas over the course of a year and that this 2 mpg increase represents a 7.7% increase (from 26 combined mpg to 28), it would take about 7.67 years to make up the $1,712 (and that assumes that the interest rate is zero).  A 3 percent interest rate will make it take about 10 years to pay this back.  Any repair costs would add to the number of years it would take before one breaks even.

From the Pittsburgh Post-Gazette:

On the nonhybrid side, the poster car for conventional fuel-efficiency excellence is Chevrolet's own Cruze Eco, which returns a government-rated 28 m.p.g. in town and 42 on the highway without any electrical assistance. Instead, the Cruze Eco uses a small turbocharged engine, a manual transmission and old-fashioned tricks like lightweight forged wheels.
The Malibu Eco is a larger, more powerful car, but its mileage numbers are a long way off the nonhybrid Cruze, at 25 m.p.g. city and 37 m.p.g. highway. The Cruze Eco even has more trunk space than the Malibu Eco because there's no battery pack beneath the carpeting.
While the Environmental Protection Agency hasn't released final numbers for the new base Malibu, I can't imagine that the Eco will beat it by more than 2 m.p.g. For comparison, the old 4-cylinder Malibu netted a combined city-highway rating of 26 m.p.g., while the 2013 Eco returns 29 m.p.g. combined. The Camry Hybrid beats both Chevys with a combined rating of 41 m.p.g.
Even the 2013 Nissan Altima, with a conventional nonhybrid powertrain, improves on the Malibu Eco by 2 m.p.g., with a combined city-highway estimate of 31. . . . .