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I hate to tell you this, but such numbers aren't really going to help. NGK are supposedly better. Unless you have a dyno and care to burn thru 3 tanks of gas per plug, you're probably not going to get any consistency in your numbers. Temperature, hills, traffic, barometric pressure, acceleration, route, etc will all make huge changes in your numbers.
Yep I will agree with that. In fact I was a bit pissed with getting 19/20mpg with light driving and no flooring!!!
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But in all honesty, I would usually get 18MPG with my old NGK's. Now this is 18MPG with half city half highway and redlining the car like 20 times throughout the tank.
With all the differences that can happen, I tried to drive only about 50 miles, now this does leave a lot of room for error (like redlining the car once will throw the numbers off big time), but this left less room for error with atmosphere etc.
I now am using the NGK Iridium plugs and have them indexed, and I think they are doing much better.
Here are the rough numbers
I drove 17 miles in the city, with the car seeing the redline about 6-8 times (I had to get it out of my system from not redlining the car for the above experiments
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The needle was at the first quarter mark at 90 miles. This is fairly good compared to what it was in the past, especially since I redlined the car so many times.
So if I'm lucky the car was getting about 33mpg total
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Now I even did my best to leave the oil as is and not change it, etc.
Now I am keeping a log, spreadsheet, of all the gas receipts and miles driven. I'm also listing the mods and changes to the car and weather.
This way I can keep a log of when the MPG's dropped/rose and why.