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Old 11-12-2009, 03:02 PM   #1
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Scan Gauge Fuel Offset Questions

I've been running a SG for awhile now with good results. However, I seem to need the collective assistance of the memebers to assist me with sorting out a perplexing dilemma and increase the accuracy of my readings. When I fill my tank I adjust the SG to match the gallons pumped and the fuel offset percentage changes, sometimes dramatically. This in turn effects the MPG readings of the SG. I've listed the figures for the last few fill ups in the hope it will assist you. And, as always, thank you in advance for your assistance.
goldie

Scan Gauge Fuel Tank
Tank - 599 miles 606.3 miles (odometer)
22.9 gal. 22.3 gal.
5.3% offset

Tank -538 miles 549.6 miles
20.7 gal. 21.6 gal.
8.8% offset

Tank -500.3 miles 533.6 miles
20.8 gal. 19.7 gal.
3.9% offset

Tank - 562 miles 569.1 miles
forgot to note gallons 22.3 gal.
11.1% offset
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Old 11-12-2009, 03:14 PM   #2
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You didn't list any figures. You can see some of my offset percentages in the gaslog for Effram.

If it's changing dramatically, you may not be filling accurately. The most accurate way to fill is:
- At the same pump
- On the same side of the pump
- With the car facing the same direction
- Make sure the nozzle is inserted fully into the car each time, and if it has a boot make sure the boot interfaces with the car the same as always
- Hold the handle at full pumping speed until it clicks off
- Don't top off

Of course, that's not always practical, but I try for that.
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Old 11-12-2009, 03:37 PM   #3
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Here are the figures I neglected to include.

Odometer Reading - 606.3 miles, 22.3 gal.
SG - 599 miles, 22.9 gal., 5.3 offset

Odometer Reading - 507.8 miles, 18.8 gal.
SG - 502 miles, 19.6 gal., 4.5% offset

Odometer Reading - 549.6 miles, 21.6 gal.
SG - 538 miles, 20.7 gal., 8.8% offset

Odometer REading - 533.6 miles, 19.7 gal.
SG - 500 miles, 20.8 gal. 3.9% offset

Odometer Reading - 569.1 miles, 22.3 gal.
SG - 562 miles, forgot to note gallons, but did note an offset of 11.1%
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Old 11-12-2009, 03:51 PM   #4
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That kind of variation is pretty common. I got tired of constantly adjusting my mileage, and being disappointed at my next fill-up. Finally, I selected the worst-case calibration figures for each of my family's cars, and use that figure all the time. At worst, the SG is accurate. At best, I get surprised with excellent results better than predicted by the SG.
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Old 11-12-2009, 03:55 PM   #5
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That range matches what I've observed with my car.
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Old 11-12-2009, 05:25 PM   #6
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You could have a combination of pump measuring error and you may not be filling it up consistantly to the full level of the tank. Best to fill the tank to the point that it spills out or just below that then run that tank and do it one more time and set the fuel adjustment and don't change it from that point on. I find that when I fill up I get within 0.1 gallons of what the SG reads with the usual variations if I don't really fill it to the top. Once in a while you will get more gas from the pump, filling the tank sooner than the SG reads . . . keep filling up at that station and pump since you could be getting some free gas!
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Old 11-13-2009, 09:58 AM   #7
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My distance is off by about 0.5% high. I had to increase it by 1%, because it originally read about 1% low. After my initial calibration, I was disappointed that it took 0.4 extra gallons to fill it versus what the SGII calculated. I will see what it looks like this time.

I do like that I'm averaging 34 mpg right now, and I have had some trips over 36. My car's 2008 city/highway/combined ratings are 22/28/25.
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Old 11-13-2009, 03:48 PM   #8
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If you adjust your speed on the SG with an accurate measuring device ie gps or radar your distance will be correct. Then it is just a matter of getting an accurate gas pump or at least one that gives extra gas.
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Old 11-13-2009, 04:43 PM   #9
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Quote:
Originally Posted by Fetch View Post
I was disappointed that it took 0.4 extra gallons to fill it versus what the SGII calculated.
IMHO, if you're within 1/2 gallon variance you're doing good. To accurately calibrate the speed the SG manual says to switch units over to KPH, and switch your GPS to KPH. Drive down the highway, and have a passenger adjust the ScanGauge until the readings match on the SG and the GPS. Save the setting, then switch back to MPH.
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Old 11-14-2009, 06:16 PM   #10
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Thanks for all the information. I'll put it to good use!
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