furnace filter efficiency impact

I've read in many places that you should change your furnace filter regularly as a dirty filter will lower the efficiency of your furnace.
Excluding extreme cases where a filter is exceptionally dirty, how much of a loss in efficiency are we talking about for the average "needs to be changed in the next couple of months" filter?
PS: I'm not trying to find a reason to stop changing my furnace filters or anything. I'm simply curious as to how much impact a dirty filter can have. Is it 0.1 - 2% or more like > 10%?
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I don't have a number for you, but you may want to cosider this.
If your filter is blocked, then less air is being fed into the furnace by the cold air return ducts. If you don't have a perfectly sealed furnace chamber, then the furnace might be drawing some of it's air from your basement, which might be pulling air from the first floor dwn to the basement. This might increase the mold growth in your basement (i.e. bringing relatively warm moist air into the cold basement).
Just a thought.
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Jonny R wrote:

Why don't you experiment yourself? I saw a clogged up filter bringing down room temp. to near freezing point in dead winter. No kidding. When I took the filter out, it looked/felt like a black sheet of slate. Tony
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