A new report from JD Power indicates hybrid cars will clock 3.5 percent of US vehicle sales by 2012, sevenfold higher than their current 0.5 percent of the market in 2004. In the same period, diesels will grow from 3 percent to 7.5 percent of sales. The number of available models in both categories are also expected to climb, with diesel models doubling from 14 to 26, and hybrid models increasing fourfold, from 10 to 44. While such figures indicate neither a European-style diesel boom nor a dotcom-style Hybrid upsurge, to most industry watchers they represent dramatic growth. The largest factor holding back the numbers? Cost premiums on production of both engine types.