In reality most spark plugs deliver the same power. In fact most new style plugs with platinum (and any non-copper tip) deliver slightly colder spark.
So if want to get hot spark, you can go with copper tip. But the diff is so little, I would stick to the new non-copper tip as they last about 100K miles.
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As for which is better (or best), again not much difference. If you want to get better MPG, just do follow the old traditional advice: First get a tune-up.
(1) spark plugs
(2) spark plug wires
(3) distributors, coil, rotors (if you have them).
(4) new air filter
(5) PCV
(6) correct tire pressure
(7) clear the trunk of useless heavy items
(8) spray clean MAF sensor.
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Forget all those "SAVE GAS" scams. This was on NBC morning TV show. The difference is as much as 38% (their claim). There are very simple rules to save gas:
(1) drive slow - not too much past 55
(2) accelerate slowly
(3) don't brake unnecessarily
(4) use cruise control - whenever it is safe to do so.
Don't coast - it is NOT safe (to coast in Nuetral) and it does NOT save gas. It only saves gas, if you are planning to slow down (exiting, tolls, traffic). Coasting then accelerating again is LESS efficient than just cruising.
The above really works. Using those simple rules I was able to drive from Boston to NYC (210.5 miles) on just 5.112 gallon (94 Civic). That's 41 MPG (94 Civic 187K miles). Actually I was speeding at 72 MPH so I could do even better at 55.
WOW !!
Good Luck....