After 63,789 miles I finally retired (sorry) my OEM Michelin MXV4 Energy tires last spring, replacing them with Michelin MXV4 Primacy in the same size. The Energy tire was designed with a low rolling resistance to help provide higher fuel economy. Primacy is similar in most respects except for the trade off in rolling resistance. Since I switch to Hakkapelitta RSi snows every winter, I'm also reporting their fuel economy results here. Hakkas in general are reputed to be lower rolling resistance than average. This is certainly confirmed by my data.

Energy 31.39 mpg in 63,789 miles
Hakka Rsi 31.32 mpg in 15,415 miles
Primacy 29.63 mpg in 5,610 miles

So far the regular grade Michelins are providing about 5.5% worse fuel economy than either the Michelin Energy or Hakka RSi. At $3.50 per gallon over 60,000 miles, this means the Primacy will end up costing me about $400 more than would a set of Michelin Energy tires solely due to fuel economy differences. If they weren't the same size and model (MXV4) from the same manufacturer, I'd wonder about circumference differences. So it must be the rubber itself.

The MXV4 Energy tires really seem to provide significantly better fuel economy. As for other differences between them, with my sedate late middle-aged driving style, frankly, I can't tell any difference. My car is a 2003 S60, manual transmission, non-turbo. Tires are 195/65/15. I keep them pumped up.

There are a few articles on low rolling resistance tires on the web -- easy to find through Google.