I changed my spark plugs to Denso Iridium a few months ago and I notice the gas mileage has gone down from the old OEM plugs as soon as I put the plugs in. Does anyone know if this is normal?

 

same thing happened to me, i went from like 25mpg now to around 20mpg...same daily driver too

 

There is a simple solution to this, put the old plugs in.

Don't know why your mileage went down but maybe you bought the wrong type of plugs for your vehicle. If it's the right type then just say screw it, put the old plugs back in, and save your money that you would spend on the iridium plugs and get some more parts!

 

that IS strange...perhaps Iridiums are more sensitive to gapping?

Perhaps the iridiums are supposed to be gapped diffrently than OEM... And that info isn't getting down to the Autozone parts counter guys...

 

Ngk's all the way! :]

 

did you make sure to gap the plugs? i know iridiums are a serious pita to gap and it IS a bit dangerous to gap them without a specific gapping tool because of how hard the electrode is, but if you are running too wide a gap you'll be throwing a lot more fuel to achieve the same amount of combustion. pull a plug and see what color it is. if it's white then your problem is simple: wrong gap. if there aren't signs of running rich, then your problem is something nastier most likely.

i personally run ngk coppers in my wrx. like a fool (a fool who'd been up for 48 hours straight) i forgot to gap my plugs when i first installed them and managed a max of 16mpg. one hour and 4 gappings later i'm back up to normal.