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MPG: computer vs calculated ???

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Old 04 Apr 2012, 10:48 am
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Default MPG: computer vs calculated ???

I am just getting dialed in to my new-to-me '06 GT convertible (35K miles)and I am a little confused about the MPG reading on the dash, it seemed quite low. only 23.6 cruising up I-5 for 400 miles (OK I played with the turbo just a little.
So on my last tank, which was mostly normal driving , fairly gentle around town, I reset the trip, reset the mpg computer and filled up at my regular station. After 227.0 miles I returned to the same station and filled up and topped off just like before. My dash computer at this point read 19.9 MPG (It was 20.0 just before I pulled in and waited about 4 minutes in line). I took exactly 10.311 gallons. My math shows that as 22.015 MPG.
Can the computer be 10% low??
Has anyone else made this comparison??
Afterwards I paid attention to my Tripodometer vs the highway mileposts and got a trip reading of 24.3 miles when the milepost showed 25.0, meaning the trip reads 2.8% low, but then I assume the computer and tripodometer are reading the same anyway so it should not effect the relative accuracy. My speedometer reads the same as my GPS.
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Old 04 Apr 2012, 01:22 pm
UptownSport's Avatar
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Default Re: MPG: computer vs calculated ???

Tires are much taller than OE?
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Old 04 Apr 2012, 02:16 pm
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Default Re: MPG: computer vs calculated ???

not really, Tires are 235/40R 18. Rotations Per Mile = 795.
Orig. equip is 205/50R 17, R.P.M. = 804
so these are only a slight bit more in circumference.

That was my first thought so I checked the mileposts vs trip. That came out the opposite of what I expected.

...too much math, my head is starting to hurt...
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Old 04 Apr 2012, 03:43 pm
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Default Re: MPG: computer vs calculated ???

well you will see a lot of guys use "FULLEY" to keep track of mpg's I did for awhile but now i just fill up mu readout and the "REAL" numbers are a lot like yours It will say 21.3 and i will actually be getting 22,5---its close enought for me so i jsut go with it
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Old 05 Apr 2012, 12:00 am
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Default Re: MPG: computer vs calculated ???

Ah, but how big is a gallon? US Liquid, US Dry or Imperial?

The gallon (abbreviation gal)is a measure of volume. Historically it has had many definitions, but there are three definitions in current use: the imperial gallon (≈ 4.546 L) which since metrication is used colloquially in the United Kingdom and semi-officially within Canada, the United States (liquid) gallon (≈ 3.79 L), and the lesser used U.S. dry gallon (≈ 4.40 L). The gallon, be it the imperial or U.S. gallon, is sometimes found in other English-speaking countries.
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