@fzaman
Always mention vehicle make and year
Every Spark Plug has its own recommended Gap settings but mostly company designated Spark Plug settings for that specific engine should be used.
Keeping the gap short will give you better feul economy and pick.
MY OWN PRACTICAL TEST:-
I bought Autolite AP-3923 Platinum Plugs for my Daewoo Racer 1993.(Before changing to brands other than company recommended one`s,always check plug specification--most important resistance or non-resistance ones as by putting non-resistance type Spark Plugs in vehicles,where recommended plugs are resistance type, you may damage your distributer coil or short the Spark Plug Wires).Recommended settings for this type of plugs on the net was 0.35mm-0.45mm.I used 0.40mm settings and drove the vehicle(improved acceleration esp lower gears and smooth idle engine) but my vehicle CO settings went to 3.93 & i couldnt bring it down below 3.86.I then readjusted Gap setting to 0.63mm(deawoo Racer 1993 1.5 SOHC engine G15SF setting by mechanics is 0.80mm).I felt no difference in acceleration or engine sound then and when the setting was 0.40mm and i brought my CO setting to 3.00.before this the engine would start missing below 3.84mm setting.
I did the same with my Suzuki Alto 800cc.Recommended setting for alto is 0.80mm by mechanics.I adjusted the Spark plug(Standard ones) setting to 0.45mm but the car would konk out on idle state with excessive missing.Tried 0.55mm some improvement but end result was still same.Then after shifting to 0.63mm setting the vehicle idle state became stable and smooth and there was 20% increase in acceleration & 10% feul conomy)
My point is:-
1.Find out your vehicles manufacturers recommended Spark Plug setting
2.Adjust Spark Plug gap some 15mm lower than the recommended setting and try it.Shorter the gap, better Acceleration but too short gap will lead to loss in feul economy at rate of increase in acceleration.(rarely can lead to engine cylinder damage)
