[aardvarcus]

In broad generalizations I believe you are on the right track, assuming the less efficient cars are less efficient because they have higher aero drag due to the body not the tire, more non tire related drivetrain drag, etcetera and also assuming you are going from a "normal" non LRR street tire to a "normal" LRR street tire.

However, there are many instances which defy these rules. For example, compare a truck on 33" 285/75/16 mud tires, versus 31" 245/75/16 LRR tires. Most people report 1-2 MPG difference when making this kind of switch. (All too often reporting it going the wrong direction...) In a pickup truck that only gets 12 to start with, you are talking in the neighborhood of a 8-16% change.

Edit: The statement about the savings in the article is “The fuel saving dollar value” not the percentage mpg difference. Also don't forget to take into account fuel useage is inversely proportional (gallons/mile) to miles per gallon, thus the actual fuel cost impacts for making any improvement will be a lot more substantial on a less efficient car. The extra mpgs help the fuel bill a lot more on a lower MPG car. (Assuming the same distance is driven.)