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With my 03 Sierra I notice that when I take mileage, that the actual mileage and fuel consumption is not accurate as measured on the computer. When filling up/taking mileage there is usually a .5 to .8 of a gallong descrepancy. I filled up tonight and the computer said I used 20.5 gallons, and I actually put in 21.3. Accordingly, the MPG is off too. Actual mileage was about 17.4, while the computer said 18.2. Is this typical? It seems off by quite a bit, but what do I know, maybe that's just the way it goes with this high tech stuff. Is there a way to adjust the computer? Incidentally, the mileage I got included some highway driving, but also short trips in below zero weather and a fair amount of idling time. Thanks in adance. Paul
 

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I tend to trust the DIC. There is a lot of variation in how the tank is filled. The trip off on the nozzles is easily off by the amount you described. Of course, the DIC is dependant upon the fuel metering device in the truck but it is fully capable of being as accurate as the meter in the pump.
The best way to compare the two is to run the average over several fills. Let's say you reset the DIC and fill the tank. Let the DIC run for five tanks and keep track of miles and fuel used over that time. Then run a comparison. Chances are you will be very close.
Dave
PS: I tried this on the 2003 2500HD I had and the DIC was dead on with the manual calculation.
 

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That is typical of the way mine reads and always that direction.
 
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