Spark plugs
 

My cavalier has been having some decline in FE lately so I started giving her a tune up. Once I popped her plugs out I was surprised to see that two of them looked like that had been in her years before I bought her. One was almost burn through. I can not imagine how it was even sparking.

Now the question I have is 2 fold.

How big of an effect on fuel economy does new plugs have? (1-3% is big enough for me)

What would be the best replacement? I was reading about Denso TT plugs and they said they improve FE, but so did a set of Bousch +4's.

Platinum +4's are kind of a waste. The +2's made noticeable effect in my '97 Escort though. I'd say if your plugs are that bad, new ones will probably bring a 5-7% increase.

Depending on their condition, smoked plugs can have a big effect on FE. When my wife and I first got married (2003), she had a 1993 Ford Explorer. Within a few months of marriage, I asked her to start calculating mpgs since it can be a half decent (and easy) way to see if a vehicle needs maintenance. It was getting 15-17mpg. Now, understand, her parents are not exactly mechnically inclined. I mean, they took the thing to the dealership for oil changes. I made the assumption that the vehicle was well taken care of.
Well, about six months later, she had a tank where it dropped to about 11mpg. It also ran a little rough. Okay, I ask, "When was the last time the truck had a tune up? It's got better than 125k on the odometer." My wife's response: "Um... I'll ask my dad." Her dad: "Tune up? You know, I don't think we've ever gotten one."
Long story only slightly longer, I pulled the plugs out. They were the original Motorcraft copper plugs, you know, the ones that they recommended replacing every 30,000 miles. Of the six, three of the plugs were so far gone that the ceramic above the threads was blackened and broken. Six new single prong platinums equated to around 17-19mpgs and a lot better torque.

Yeah, it all depends. I would think if they're as bad as you say, you should notice a difference. Maybe not much in FE but likely in driveability which that alone can be worth it.

I've tried specialty plugs and they've all been a waste and in one instance it was a detriment. NGK coppers have always worked great for me in my foreign cars.

You can spend $15 a plug or buy 5 plugs for $3 each. I guess I would go with the latter as it tends to keep you busy with maintenance (which allows you to notice issues much sooner) every 30-40K rather than 100k or more. I wont say that quality plugs dont make a difference, but how long are you willing to keep them in? Do you want to pull threads out in 100k miles because they were never replaced? Buy plugs at autozone, carquest, napa, or the such (ask them for the standard plug) and get some antisieze and dielectric grease.

Goto RockAuto.com, and find a set for your car there. I just put NGKs, normally 6-8 bucks per plug, were about 6 bucks for a set on the CRX. Factory quality, and came pre-gapped (I didn't have to adjust any of them). AC Delco is always good as well, about the same price if I recall right.

Either way, the prices of plugs with shipping will be cheaper than what any parts store could give me for those equivalent plugs. Parts get to you in 2-3 days, if it's not in one of their "other" warehouses (it will say in the item description if there will be a delay).

New plugs can have an effect, rather significant possibly... definitely change them. They're cheap enough.

You drive a Cavalier... It's a Chevy, put what the manufacturer called for which is probably AC Delco plugs...don't waste your money on something not meant for your engine.