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TDI Fuel Economy Discussions about increasing the fuel economy of your TDI engine. Non TDI related postings will be moved or removed.

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Old February 8th, 2018, 05:37   #16
cbman18j
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I won't rule out something is wrong but everything that I read about vehicles at sea level that run in excess of 70 mph for extended periods of time seem to get mpg in the low 40s without a tune. I wont say how much in excess I run above 70 but the fuel economy I saw with the 15" avus was what I had been expecting giving my driving conditions and road conditions. Just as a note I have gone thru the entire vehicle over the past 2 years and 100k miles you can see in some of my other threads,
intake cleaning, fuel filter, air filter, front/rear brakes, even a stuck caliper, all fluids, timing belt, vacuum lines, turbo actuator, timing adjust to green in vagcom.
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Originally Posted by IndigoBlueWagon View Post
I wonder if something's off with your current FE calculation or with your car. Although what posters say about wheel weight and tire width is correct, I can't see it making that big a difference in FE in the same driving conditions. And to me it's a red flag whenever any manual transmission ALH shows less than 40 MPG. Seems to me that all your MPG numbers are a bit on the low side.
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Old February 8th, 2018, 11:28   #17
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Check the MAF sensor as well. I noticed my MPG has dropped off and the power didn't seem to be there. I cleaned it last night and power is back, plus the exhaust drone that cropped up has gone. We'll see if the MPG come's back.
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Old February 8th, 2018, 18:18   #18
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I have recently replaced the MAF also.
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Old February 10th, 2018, 08:33   #19
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However, the 16 inch tyre is rated 69 decibels, while the 15 inch tyre is rated 67 decibels. That might not sound much difference, but decibels is measured in a logarithmic scale, so a difference of 1 decibel is considerable.
Actually, it's not considerable: 1 dB is the smallest difference in sound pressure level that can be detected by the human ear. The 2 dB between these tires may well be barely perceptible to most people.

In truth, the dB difference is pretty much meaningless without discussing the frequency of the sound. Humans perceive much smaller dB differences around certain frequencies and don't perceive much larger dB differences around some others. So it's not just the sound pressure level but also the frequency composition of the sound that affects how we perceive the sound.
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Old February 13th, 2018, 07:37   #20
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Actually, it's not considerable: 1 dB is the smallest difference in sound pressure level that can be detected by the human ear. The 2 dB between these tires may well be barely perceptible to most people.

In truth, the dB difference is pretty much meaningless without discussing the frequency of the sound. Humans perceive much smaller dB differences around certain frequencies and don't perceive much larger dB differences around some others. So it's not just the sound pressure level but also the frequency composition of the sound that affects how we perceive the sound.

No its not meaningless.
We are not talking about perceived sound levels to the human ear (Psycho-Acoustics). We are talking about measured sound pressure (energy) as measured by the tyre manufacturer. This is about fuel economy when changing rim sizes.
We are then talking about 4 tyres, 4 times the change.


A Bel is an order of magnitude (10 times) of power and a decibel is one tenth that. Thus, 1 dB is a power change of about 26% (10^0.1). Frequently decibels are referenced to a standard power level. For example, dBm means dB referenced to a milliwatt where 0 dBm is one milliwatt, -10 dBm is 0.1 milliwatts, +20 dBm is 100 milliwatts, and so forth. This means each decibel is a different amount of watts depending where on the scale you are. At 0 dBm, one decibel means about 0.26 milliwatts but at +30 dBm (1 watt) one decibel is 0.26 watts.

For sound levels, the same kind of thing is true. 0 decibels SPL (Sound Pressure Level) is a specific amplitude of sound level (yes, there can be negative values). Going from 50 dB SPL to 51 dB SPL requires a lot less watts than going from 100 to 101 dB SPL.
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Old Today, 07:59   #21
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*shrug* I guess you're the expert. Wasn't clear to me we were not talking about psychoacoustics, though that is a (minor) issue for vehicles. I stand corrected.
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