My '99 accord LX (4 cyl) has 95K miles. I changed out the spark plugs for the first time 3 wks ago with NGK brand, copper. The model I purchased was the exact same as the one recommended in the manual. I also changed the engine oil at the same time using Mobil 1, 10W-30.

I am now seeing a significant drop in the MPG. Over the past 5 years I was getting consistently 450 miles per tank (freeway miles), but over the previous two tanks since the plug change, I am seeing 325-350 miles on the tank. Huge difference!

Is this due to the spark plugs not being gapped correctly? I thought the NGKs are pre-gapped according to factory spec. Also, if I switch the plugs to the Iridiums, will that solve my problem? I read they are designed with a more forgiving gap distance.

btw, my air filter is only 2 months old, and the tire pressure is normal (34psi). my friends say the mpg drop could be caused by the oxygen sensor, fuel filter, spark plug wires and/or distributor cap. but i think the main culprit are the plugs.

Any advice will be just great...

 

some ngk plugs come pregapped, but i would check them just to be sure

when was the last time you did maintenance on your car?