[topcat]

Last weekend I replaced the plugs on my Toyota 1ZZ-FE engine, after 66k miles. I replaced like for like with standard NGK copper plugs BKR5EYA-11. These are supposed to have a 1.1mm gap.
I went through all the decisions, like surely if it is not misfiring, then don't bother changing them. Surely a longer spark gap from the old eroded plugs would give better economy, due to more complete ignition.
My decision to change them was mainly as you say guilt! And also to give the coil packs an easier time, and thus less likely to burn them out.

Well, after 66k miles, the gap on the old plugs was more like 3mm! As I replaced them, I made a note to index the plugs, i.e. face the open side towards the air intake manifold if possible (front of car on _this_ engine- back of car on most other engines). This was luckily where 3 of the 4 naturally ended up. Unluckily, the 4th one ended up facing the exhaust manifold with the correct torque wrench setting, so I gave it an extra 1/4 turn and it ended up side on, 90 degrees out. The new plugs, I set to the manufacturers recommended 1.1mm gap.

I am sad to say that my fuel economy is slightly down after doing this. My 20 mile trip to work usually gives 54 MPG (imperial) but this week I have got about 52 MPG.

So my conclusion would be, for best FE, use the longest spark you can before misfires occur. But for longevity of coil packs, use the recommended spacing. A trade-off as always.
Tom