According to the latest forecast by J.D. Power and Associates, sales of hybrid and diesel-powered cars will more than triple by 2015. The new study predicts that hybrids will account for seven percent of the car market in 2015—up from 2.2 percent in 2007— and diesel vehicles will carve out a 10 percent market share as compared to last year’s 3.2 percent.

Historically, J.D. Power has offered one of the auto industry’s most conservative hybrid forecasts. For years, the company has also forecast higher penetration for diesel vehicles compared to hybrids—a scenario that has not yet materialized.

Forecasting methodologies require listening closely to statements by auto executives, but not necessary taking them at their word. Anthony Pratt, former senior manager of global powertrain forecasting at J.D. Power, told HybridCars.com in 2005, “A fair amount of grandstanding takes place by vehicle manufacturers.”

Accounting for hybrid vehicles in development, but perhaps years away from dealerships, is tricky business. For example, the latest J.D. Power hybrid study includes significant sales numbers for the Chevrolet Volt. According to John Teus, spokesperson at J.D. Power, the Volt is forecast to sell 11,059 units in 2010. Teus told HybridCars.com, “Our sales forecasts take the Volt from 58,700 units in 2011 to 70,000 in 2014.”

The latest numbers from J.D. Power show an upswing in smaller four-cylinder engines over larger gas-thirsty powerplants. Smaller car sales are expected to make up 35 percent of the market in 2015, up from 30 percent in 2007.

J.D. Power evaluated other green alternatives to powering automobiles, like hydrogen fuel cells and electric vehicles, but expects these technologies to have a negligible impact for many years.