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Query on erratic gas usage.

899 Views 6 Replies 4 Participants Last post by  jimvw57
I've mentioned before my glee at the excellent gas mileage from my 2017 Smart Prime cab. Substantially better than my 2013 was! I just drove the 1 1/2 hours from Bloomington IN to Indy averaging 54 mpg.

But the mileage varies in unexpected ways. I sometimes drive about 25 miles on a particular drive through isolated state highways with mild hills, 40 - 45 miles her hour on the average. I average 48-50 mpg.

I did so one weekend, then on the following weekend, in identical weather, identical wind conditions and the same tank of gas, I averaged 37 mpg. Even the same time of day. I can't isolate a single defining difference that might easily account for this difference. During the following week, on a new tank, mileage remained low.

Any helpful thoughts?
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Exactly why gas mileage should be averaged over multiple tanks of gas. some pumps deliver fuel at a faster rate and cause the gas to foam and fool the shut-off into thinking it is full when it isn't. thereby looking like you got more miles on less gas, the next tankful takes more fuel and makes it look like you used more gas on less miles.

Even using the same pump, at the same time of day, same temp, and same route can be affected by humidity, barometric pressure, wind speed, temp of the tires, pressure on the gas pedal, weight in the car, etc. There are also variables inside the motor and transmission that can affect MPG as well. Averaging over at least 3 (or more)tank fulls will give you a better indication.
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I did monitor my first car religiously for 167,000 miles and the wife always seemed to get in the 53 range on drives up to the cabin, while I could get 50 ish occasionally, but mostly in the upper 40 range. Still better than the 15-17 MPG in my Expedition so I can't complain.

Truth is I got this car for it's low purchase price and the fact that it is fun to drive. The MPG is just an added bonus!
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well over 3\4 of my miles are to work and back. same route, same station for filling up, same method. variables are weather and temperatures, and traffic. Most of the time, there isn't any traffic as I work 2nd shift. I do see a fluctuation in mpg occasionally but I just don't worry about it.
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