Wow! New Plugs = +20% Better Mpg

rt66er · April 4, 2011

I bought this truck last June (91,850 miles on it) and it has consistantly averaged 15.8 to 16.3 mpg. Changed the spark plugs about a month ago. First quarter tank (before change) showed 16.1 mpg; by the time I was down to last quarter of that tank, DIC MPG had increased to 17.4 mpg. Filled up yesterday and it now shows 19.8 mpg so far. I doubt the mileage will stay that high for this entire tank of gas, but it still is showing a significant (~20%) improvement in mileage. Best thing is that it didn't cost me a penny. I had a set of AC plugs that I pulled out of a 2004 GTO back in 2005 that only had about 1,000 miles on them. Checked and, sure enough, exact same part number that the truck uses. Wish I had done this last summer! Just goes to show: it's a good idea to change those plugs long before 100,000-mile recommended interval.

Superperv23 · April 4, 2011

Fresh plugs can make a good difference, everyone is so concerned with long service intervals these days that parts need to be rated for many miles any they will last but they will not work perfectly until then so if you want top performance from your engine you have to change them more often.

ChevyTech77 · April 4, 2011

My current truck has about 100k on the plugs and a year on the K&N air filter. I suspect that by changing the plugs, and cleaning the air filter, my mileage will pop back up the 2 mpg that it has lost over the 6-8 months or so, just like it did back when I last changed the plugs at 98k.

cujo8 · April 5, 2011

I bought this truck last June (91,850 miles on it) and it has consistantly averaged 15.8 to 16.3 mpg. Changed the spark plugs about a month ago. First quarter tank (before change) showed 16.1 mpg; by the time I was down to last quarter of that tank, DIC MPG had increased to 17.4 mpg. Filled up yesterday and it now shows 19.8 mpg so far. I doubt the mileage will stay that high for this entire tank of gas, but it still is showing a significant (~20%) improvement in mileage. Best thing is that it didn't cost me a penny. I had a set of AC plugs that I pulled out of a 2004 GTO back in 2005 that only had about 1,000 miles on them. Checked and, sure enough, exact same part number that the truck uses. Wish I had done this last summer! Just goes to show: it's a good idea to change those plugs long before 100,000-mile recommended interval.

Another possibility is the gas you're now using may have changed to the "summer" blend, which will give you better MPG's. I have a 2011 GMC Sierra with the 5.3L AFM engine that is supposed to get 21MPG on the highway and the best I've been able to get on the "winter" Blend gas was 17.6MPG. I just filled up last Friday and it seems that my fuel economy is starting to improve, since the DIC say's I'm getting 19MPG average right now. I am suspecting that the station I get my gas from may have switched over to the summer blend, but I can't say for sure since the truck doesn't even have 1500 miles on it yet and I've heard that milage can improve after a certain break-in period. Changing the plugs on a higher milage engine is always a good idea regardless and better MPG's are also welcome to say the least. FWIW, I saw a segment on HorsePower TV where they dyno tested a new performance engine with stock plugs and then again with the new e3 spark plugs and they showed measureable performance gains with the new e3 plugs ,but remeber your milage may very.

rt66er · April 5, 2011

Another possibility is the gas you're now using may have changed to the "summer" blend, which will give you better MPG's.

I understand what you're saying. Keep in mind, though, I'm comparing mileage over a nine month period, not just a few weeks. Pretty much encompasses all seasonal blends from early July to late March. In addition, I buy all my gas from one station (it's the closest to my house, and it's owned by a relative) that sells only gas with zero ethanol content. I wish I had saved the old plugs so I could check the gap; I'm guessing they were pretty far out of spec.

txab · April 5, 2011

Another possibility is the gas you're now using may have changed to the "summer" blend, which will give you better MPG's.

I understand what you're saying. Keep in mind, though, I'm comparing mileage over a nine month period, not just a few weeks. Pretty much encompasses all seasonal blends from early July to late March. In addition, I buy all my gas from one station (it's the closest to my house, and it's owned by a relative) that sells only gas with zero ethanol content. I wish I had saved the old plugs so I could check the gap; I'm guessing they were pretty far out of spec.

Seriously doubt it. Unless they were gapped improperly when installed. I'm looking at a random sample of 9 used plugs out of 3 trucks with over 100k on each set, still right on the money.

pistonslap · April 6, 2011

Another possibility is the gas you're now using may have changed to the "summer" blend, which will give you better MPG's.

I understand what you're saying. Keep in mind, though, I'm comparing mileage over a nine month period, not just a few weeks. Pretty much encompasses all seasonal blends from early July to late March. In addition, I buy all my gas from one station (it's the closest to my house, and it's owned by a relative) that sells only gas with zero ethanol content. I wish I had saved the old plugs so I could check the gap; I'm guessing they were pretty far out of spec.

Seriously doubt it. Unless they were gapped improperly when installed. I'm looking at a random sample of 9 used plugs out of 3 trucks with over 100k on each set, still right on the money.

It is quite possible that the gap had increased from the wear on the plugs! What i am going by is my experience when I changed my plugs at 100,00 miles. My truck I bought used, so I don't know for sure if the plugs were original but by looking at them id say they were. My gap was way wide compared to the original gap. My truck ran fine with the old plugs with the excessive gap. Even with the new plugs I never noticed any significant increase in power or mileage. On my old Ford pickup I used V electrode plugs. They worked good but i found that due to small surface area of the electrode, they seemed to wear out faster,which also resulted in wide gaps .

txab · April 6, 2011

The older plugs did have gap widening over time due to the materials they were made from. On these new Iridium plugs I'm not seeing it. The tips don't really erode. If your speaking of your 2000, it used plugs gapped at .060" from the factory. GM issued a change directive around 2003 or 2002, not sure exactly, the replacement plug gaps were lowered to .040"

Can plug gaps increase. Yes. But their are 24 of 'em here that show me otherwise, in a properly running engine. Today's plugs can run a lot longer than 100k if necessary

00Silverado4x4 · November 26, 2011

I know it's been awhile but since you've had a few months I thought to ask how the plugs are working out for you? Have you still been getting the increased mpg from your truck? In the upcoming months I'm going to be working on my truck to include spark plugs, however to be honest I have every intention of staying with stock plugs as I do burn e85 & must have the AC Delco's.

rt66er · November 26, 2011

I sold the truck in July, but the mileage improvement remained fairly consistant at about +2 mpg...

pistonslap · November 26, 2011

The older plugs did have gap widening over time due to the materials they were made from. On these new Iridium plugs I'm not seeing it. The tips don't really erode. If your speaking of your 2000, it used plugs gapped at .060" from the factory. GM issued a change directive around 2003 or 2002, not sure exactly, the replacement plug gaps were lowered to .040"

Can plug gaps increase. Yes. But their are 24 of 'em here that show me otherwise, in a properly running engine. Today's plugs can run a lot longer than 100k if necessary

I have changed plugs on my 2000 at about 100k ,and the gap was definitely wider than the originals. Replaced the plugs with iridiums. I never saw any mileage or horsepower increase. I guess this shows how hot or efficient the spark is.