[Diesel_Dave]

I haven't found any similar studies on diesels, but I'd be willing to bet that there is a FE effect on diesel. The reason being that most diesels don't have closed loop control on air-fuel ratio. On a typical gas engine, if you decrease the intake air restriction, the throttle will close down in keep the AFR the same, thereby negating the lower intake air restriction.

On most diesels, lower intake air restriction mean higher AFR and therefore better FE. That's with diesels that are naturally aspirated, or have a fixed geometry/wastegated turbo. In some cases, engine with VG turbos (such as mine) do have closed loop AFR control. In those cases, I think there's still a FE benefit, becausse the VG turbo won't have to close down as far, thereby decreasing pumping losses.

BTW, slowmover, I'm not looking at changing my filtration--only the restriction upstream of the filter. Also, I never drive on any unpaved roads.