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Query on erratic gas usage.

897 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.
Huronlad is correct. I used the dash computer and monitored the mileage for each trip separately. As mentioned, the temp, humidity, wind speed, pressure on the gas pedal, path traveled, weight in the car, etc, were as close to identical as I can imagine. And same tank of gas. But under any circumstances, it's difficult to account for the substantial 13 mpg difference.

Of course everything you say, jimvw, is correct, and I very much appreciate your comments! It seemed like the lower gas mileage continued for a tank and a half, more or less, then returned to high mileage, but outside of the aforementioned identical trips example, I cannot be certain how much these factors differed and effected things.

I've tracked 32 tanks of gas since I purchased the car using the iPhone app "Gas Cubby,", and with each fill I reset the dash computer mileage gauge and compared gas usage with that dash computer results. They seem quite similar, leading me to believe the car is correctly tracking mileage and gas usage.

What surprises me is that no other forum members seem to have indicated these types of inconsistencies.

Under any circumstances, I'm generally thrilled with the mileage I get on this car! (54 mpg traveling to the Smart Dealer in Indy from Bloomington the other day!) I'll continue watching. Unless there's serious reason for concern, I can live with occasional inconsistencies. Thanks to both who have replied!
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