2001 Corolla - Drop In Gas Mileage - Toyota Corolla, Chevy Prizm (1998-2008)

Guest Dummy

I changed to NGK iridium spark plugs around 6 weeks before when Corolla was at 70K miles (earlier than needed!).

Since then I think mileage dropped by around 30 miles per fill up for same driving habit. Mostly I make short trips within the city. I used to get 280-290 miles per fill up, now its around 250 miles. I filled up 2 to 3 times and results is same.

Not sure, if the gap is incorrect, does it make x number of miles before Spark plug settles in, incorrect installation, etc. Air filter was changed as well. Any ideas?

Nope - no need to "break-in" spark plugs. If anything, plugs run the best in their first 10K-20K miles of their lifespan.

Iridium plugs come pre-gapped - but it would be a good idea to double check them. For this generation - 0.044" gap is needed. The OEM plugs can last up to 120K miles, but not many will make it that far. Depending on your driving conditions - 70K-90K might be the limit.

If you are concerned about the plugs being suspect - put the old ones back in and see if the mileage improves.

You only replaced the spark plugs and air filter? No other changes whatsoever? Which air filter did you get, and are you still on same grade of gasoline?

Did you get NGK OE Laser Iridium IFR5T11 or NGK Iridium IX BKR5EIX-11? They are pre-gapped to 1.1 mm (0.0433") as required.

I use denso plugs but always gap them before use,have you checked your fuel filter,a blocked fuel filter will put the fuel pump under pressure and also add to fuel consumption.I drive a 2001 corolla 1.4 petrol and drive 600miles per week and will still return 53 to 63 mpg.The car has 220000 miles on it and burns 1 litre of oil per 650 miles.The car has done 614 miles to 50 litres of petrol.Im from ireland and petrol cars are fast becoming a thing of the past,most new 2012/13 cars are now small diesel engines.VW being the biggest seller with the bluemotion golf 1.6 diesel.