I asked one of the diesel mechanics here about it -
If you shift properly, there's not one tiny bit of difference using the clutch vs not using the clutch for fuel economy. note this is on big rigs, where to downshift you have to rev the engine up in neutral even when using the clutch - and rolling in neutral is a good way to get yourself killed in a big rig, so that question never came up, although the trucker term for it is Georgia Overdrive or sometimes Mexican overdrive. Gotta be on some serious medication to try that...(in a semi)
he went on to say that while you could make a case that you were causing less wear and tear on the clutch, that one shift you miss will cause 100 times the wear that a lifetime of using the clutch would cause.
I do not know if this part applies to cars - he also gave me a very strong warning against rolling down a hill in gear with the clutch depressed. He told me that due to the multiple reductions involved and no pressure on the clutch, it can actually cause the clutch to explode and send shrapnel through the cab of the truck
I'll admit to being quite guilty of not using the clutch when shifting especially in the low range gears
Dont tell anyone. lol