As stated on his website, Ernie's mission is straightforward: "To promote fuel conservation and reduce greenhouse gas emissions by researching, developing and commercializing novel technologies that simply and cost-effectively improve the efficiency of cars and trucks." Some of those novelties are the additions to the rear end, which give the Beetle a better drag coefficient and improves fuel economy by 5 to 8 percent, as Ernie explains here. By adding "lower-rolling-resistance tires, low-friction engine oil, and use of a B5 biodiesel blend fuel," the Beetle's efficiency just keeps creeping up.
Aside from Ernie's Max MPG website, you can also check out Ernie's infrequently updated blog here.
[Source: Max MPG via Gas 2.0, h/t to Thunder]