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What I do is keep the reciept and write the odometer reading and the trip meter reading on the reciept. This is the lazy [efficent] way to do it, I think. The Lifetime average is the most credable number, because it takes the total miles and devides by the total Gallons, very large sample size... I record all fuel used. In my case you can test the veracity of the data by seeing the seasional change caused by temperature... amung other things. Cheating is without any real benifit so I dought many people on this site do it, not so much as a plastic trophy for the best numbers, let alone prize money.
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