Loss of mpg after changing spark plugs

[jakewire]

My friend changed the plugs in his libby and his mpg went from 15.5 down to 13.5. I asked him what the gap was and he said the parts guy at PepBoys said they were pre-gapped already to the correct specification when manufactured. He claims it runs much better after the change especially on a cold start but can't understand the sudden loss of mpg.

Would too small of a gap cause that much of a loss in mpg?

Thanks,
Jake

 

[04liberty4x4]

It is def. possible.

I noticed this with my Honda accords. I changed the spark plugs in my newer one and my older one. Both needed plugs badly, when I changed the plugs out of Accord #1, they were .052, Accord #2's plugs were .050. WAYYYYYYYYYYYYYYYYY out of spec due to old age and too many miles on them. So i installed new NGK's(what honda calls for as OEM) to proper spec-gap of 0.044, Accord #1 was getting 32mpg on the highway doing 55mph, now its lucky to get 28mpg. Accord #2 was getting 35mpg on the highway going 55mpg, it is now at 31mpg. Both of the "NEW mileages" are within the range of the FUELECONOMY.gov ratings.

however, Spark plug gap def. affects fuel economy. I think I'm going to start going .004 over on any new spark plugs I install, the hit in fuel economy is too much to bear otherwise. My father in-law said guys used to play with spark plug gap for more HP back in the hot rodding days, but those are dead now.

my 2 cents, your mileage may vary

 

[JeepJeepster]

You should always check the gap before putting them in. They may be gaped when they are made but if someone dropped a box then it messes it all up.

Not sure if the mpg would go down that much though. Maybe he got some crappy gas or changed his driving habit some.

 

[Dave]

I agree with jeepster.

ALWAYS check the gap before you put the plugs in even though they are supposed to be right.

I have changed many sets of plugs in many vehicles and never a 2mpg loss with new plugs. Something elce is the cause and it just coinsides with the plug change. I'll bet bad gas or a switch from 100% gas to an e-10 blend that many places are using these days.

Dave

 

[jeepkj02]

Even if it is "gapped to manufacturers specs" its always better to check the gap. If its those double platinum type spark plugs those can't be gapped.

 

[jakewire]

or changed his driving habit some.

I asked him that and he said he said he hasn't been using his A/C ... LOL!

We're both off tomorrow so I'm going over in the morning and we are going to pull the plugs and see what is going on. If the gap is off I'm going to go a bit wider as the other poster responded in this thread. Why wait for good mpg when you can get it from the get-go

Thanks people,
Jake