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Old 04-13-2008, 07:40 AM   #5 (permalink)
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Join Date: Dec 2007
Location: Southern California
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1999 Saturn SW2 - '99 Saturn SW2 Wagon
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90 day: 40.49 mpg (US)
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Chris -

I would use a GPS in comparison to the car's odometer. It won't solve the 1.X % issue, but I think you will have the best possible accuracy. If you don't have a GPS, I'll bet one of your friends does.

In terms of inputing the percentage, you can always do a correction in your fuel log. That's what I do for my fuel log :

Fill date 	Distance 	Fill amount 	MPG 	
2008-04-07 	194.7 mi. 	4.946 US gal 	39.37 US 	
Ignore high/low MPG, only look at AVERAGE MPG (Chevron;87 Octane;$3.64;#9). 
Tires are larger, so odometer is reading 2% less, hence +2% 
correction: 190.9 * 1.02 = 194.718. EFIE off. 
Needed to add 1/2 quart of motor oil during this tank.

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