Rep Power: 0

Recently I put in some Denso Iridium IK20's in replacement of my stock spark plugs. The car does perform more smoothly, however my gas mileage has decreased about 3-4mpg instead of increasing by this number. I'm pretty sure I installed the plugs correctly.

Possible problems: I got some new tires on a day after I put the spark plugs in. 205/40/17 --> 215/40/17. I don't think the small amount of extra rubber could cause my mileage to decrease THAT much, could it? I run super huge sidewall tires on my stock steelies (195/65/15) and my gas mileage is usually about 32-34 still. Before I got my new tires and spark plugs in I got 32-34mpg also. When I calculated my fillup last night I only got 29.

Any ideas?

 
Rep Power: 361

so ... are you running 17" rims or steelies?

are your driving habits exactly the same?

going from 15" to 17" will decrease milage ...

 
Rep Power: 0

hm.............honestly..

that's actually the number that i got....29...

im using stock size.....

i wonder whether it is caused by the plugs or not...

have u checked ur tire pressure?

 
Rep Power: 0

Driving the same, if not less aggressive.

Tire Pressure (Hot) is at 35psi.

I'm running 17's. But I'm comparing the gas mileage to when I had the 205/40/17's.

Maybe I screwed in the spark plugs too tight, or too loose...

 
Rep Power: 811

ha finally it gets proven. Use stock double platinum! If you checked tire pressure (should be 30psi cold, never measure hot), and made sure the plugs are torqued (with a proper torque wrench) to 13ft/lbs, then its the plugs that are the problem. I've gotten my highest mpg using stock plugs.

 
Rep Power: 0

heee....gearbox seems to be happy with it. LOL

well, again, my .02..

first, re-torque the plugs to 18N-m.....

then, check ur tire pressure in cold morning, adjust it to....31psi Front, 30psi back....

ACTUALLY....i did these...and seems like it's getting a better mileage now....and less engine noise at 3000~4000 rpm.....

before, it was a bit "growling" at that range..

 
Rep Power: 0

I don't have a torque wrench... Thanks for your help guys.