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Sure I can actually get different mileage, but for the same trip in the same vehicle I don't expect more than a 5% to 10% difference between trips (which is a 1 to 1.5 mpg variation). I commonly get 2 or 3 mpg (and sometimes much greater) variations on single tank full mileage estimations. However as soon as you average over 3 tank fulls, things smooth out to the expected average.
There are just too many variable if you try to figure you mileage on a single tank full - particularly if your vehicle has a relatively small tank. It is really easy to make a 1 or 2 gallon difference in filling the tank. Maybe on the first tank full you got it really full because of the angle of your vehicle, or the pump cut-off, or your patience, or the outside temperature. On the next tank full you were at a different pump, or the car was at a different angle, or the shut off performed differently, or you were in a hurry, etc. Just say you have a 14 gallon tank and your actual mileage for the conditions is 30 mpg. If you were really careful at the initial fill-up maybe you got 15 gallons in the nominal 14 gallon tank (heck, if I wait long enough it seems I can get an extra 3 gallons in my Expedition). You drive 240 miles and actually use 8 gallons of gas. However, when you refill the car you only really fill the tank to 14 gallons. For whatever reason, you did not squeeze in the "extra" gallon. So you think you only used 7 gallons of gas and you then figure you got 34 mpg. How if you did the reverse - started out with "only" 14 gallons at a fill-up and then squeeze in an extra gallon at the next stop after 240 miles, you might think you used 9 gallons of gas and only got 27 mpg. Happens all the time. If you don't think such variations exist, the next time you fill up, drive around the block and come back to the same pump and see if you can't squeeze in another gallon or more.
For whatever reason people seem to be willing to delude themselves about fuel mileage. I have one friend with a Mustang GT similar to the one I owned. He kept telling me he got 32 mpg on the highway. I never could. I finally asked him how he was figuring his mileage. Turns out he was estimating his gas usage from the gas gauge. Or then there are the people who think they get better mileage at 65 than at 45. The problem is, they are comparing open road mileage at 65 to stop and go mileage on a street with a 45 miles per hour speed limit. Very few people are willing to get out on the highway and drive 45 mph for 200 miles to get an actual indication of there mileage at a steady 45 mph. My Expedition has a fuel mileage indicator. I participate in a mailing list with other Expedition owners. A couple of them made the claim that their Expeditions got better mileage at 75 than at 55. I figured it was bad data and set out to see what my mileage was at various speeds. To do this I used the fuel economy gage. The one on my Expedition will not give an instantaneous reading, it averages over distance. On my more or less weekly drive to my farm (145 miles away) I spend significant time on a nice relatively flat section of road (US 64 from Rocky Mount NC to Williamston, NC) with a 70 mph speed limit. I started driving this parts of this segment at various speed and recording the indicated fuel economy. I would set the cruise control to my target speed and reset the mileage meter and then record the indicated fuel economy after the mileage meter settled down to a constant reading. I did not count the results if I was forced to use the brakes or accelerator during the period. I did this over a number of weeks but avoided really windy days or rainy days (I did note that the mileage decreased on wet roads and was greatly affected by tail or head winds). I recorded the following data points for various speeds:
Speed Fuel Economy 45 22.1 45 20.8 50 21.4 50 20.7 55 20.4 55 18.4 55 19.2 55 19.5 55 19.8 55 18.3 56 21.7 58 17.9 58 20.5 60 18.1 60 17.8 60 19.2 60 17.2 60 16.2 60 21.9 61 17.8 61 19.4 62 18.6 62 17.7 62 20.7 62 16.6 62 17.6 63 16 63 17.3 64 19 64 19.9 65 15.8 65 16 65 16.2 65 18.1 67 16.7 67 16.7 68 15.7 70 16.6 70 16.9 70 13.1 70 15.5 70 17 70 18 70 16.7 71 17.3 71 16.1 72 15.4 73 14.4 74 16 74 15.5 75 16.6 75 16.9 75 15.3 75 14.1 75 13.8 75 14.5 75 14.8 75 14.3 75 15.4 76 15.2 76 16.2 76 14.6 76 16.4 76 15.6 76 14.7 78 14 78 13.4 78 13.8 78 15.5 78 12.7 78 15.2
As you might expect there appears to be a wide fluctuation in the numbers for various speeds (wind, temperature, equipment accuracy, etc), however if you graph the points out, the best fit curve is pretty much what you would expect - a decrease in fuel economy as speed increases. Here is how I see the best fit:
Speed Economy Engine RPM 45 21 1285 50 21 1427 55 20 1570 60 19 1713 65 17 1856 70 16 1998 75 15 2141
Engine Redline - 5500 rpm. 40% of 5500 = 2,200 rpm. I suppose if I had driven 80, the mileage would have magically jumped up to 22 mpg or more.
BW - Driving 45 in a 70 zone is not a good idea, but fortunately for me, the particular road is not busy when I usually make my trip. I suspect the fuel economy falls off below 45 since you can't keep the car in lock-up overdrive below 45. My technique was not perfect. It was the best I could do with the means at hand.
Ed
