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Re: ~10% is a pretty big variation, i'd say...
Posted by RhodyDave (more from RhodyDave) on Fri, 15 Jun 2012 10:58:14
In Reply to: ~10% is a pretty big variation, i'd say..., Tulsaab , Thu, 14 Jun 2012 07:58:46
Calibrating the fuel gauge is easy. Just record the gauge reading (as a decimal between 0 and 1) when you fill up, and the number of gallons required to fill. Tank capacity (16.05 gallons)- the number of gallons pumped = amount left in tank. I plotted the data as a scatter diagram in Excel, and ran a regression line thru the points. My gauge reads y = 14 x + 0.84, where y = the number of gallons remaining, and x = the gauge reading as a decimal between 0 & 1. R-squared = 0.96, a measure of how well a straight line fits the data (1.0 would be perfect).

Notice that full (gauge = 1.0, on the pin) indicates 14.84 gallons and empty (gauge = 0) indicates 0.84 gallons remaining. The reason the equation shows only 14.84 gallons when the tank is full, is because my gauge doesn't start to drop until about 1.2 gallons have been used. The data also shows that when the warning light comes on, there are between 2 & 3 gallons left.

I calibrated the SID average mpg function the same way, and got y = 0.94 x + 0.06, with R-squared = .84, where y = actual MPG, and x = SID mpg. So, my SID reads about 6% low (100 x (1 - 0.94).)

The reason I said the DTE error of 10% was to be expected, was that I'm pretty sure the calculations are done using the same data from the ECU. I don't believe that the SID has access to the fuel gauge reading. Rather, I think the thing integrates fuel consumption (from the injectors) over time to get total fuel used, and divides that by the average mpg. The integration process is error prone, anyway, but my 6% error in mpg is "on the same order" as a 10% error in DTE.

As TulsaSaab has noted, it's better to refuel at 1/4 tank to keep the fuel pump cool, especially on the V6. There have been lots of posts on various forums about the untrustworthiness of the SID DTE.

And yes, I have three engineering degrees!

posted by 100.40.11...
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