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I too kept a hard cover book in my console for each vehicle for years.
It was amazing to see the step up in MPG in May and back down in Sept./Oct every year. I asked a "carburetor engineer" that worked for Chrysler and he told me about the different formulations for winter and summer gas.
It also showed that one tank meant nothing but more than a few were harder to do under similar conditions. For 6 months I was burning about 10 gal / day and it gave me a good chance to compare a couple brands A-B-A-B etc for 10 tanks of each and found that the Silverado got 1.1 MPG better on Amoco than Speedway (YMMV).
I also noted that even with Mobil 1 and all highway driving that my mileage would begin to degrade after about 8500 miles even though the oil still looked clean and felt slick.
After I got my Impala I found that the computer's record per tank was within .7 MPG of calculated so I go lazy and quit keeping the paper record.
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