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MPG result changes depending upon level of tank at fill

1996 Oldsmobile Silhouette -This happened while I was taking a road trip and is all as highway mileage.

When I drive until the tank is near to empty I get 18-19 miles per gallon. When I fill up after going only a hundred or so miles I get more like 25-27 miles per gallon. I have checked my math and even had a friend check it. My road trip was several hundred miles long so I had an opportunity to test this a few times. Can anyone tell me how this can happen?

It has to be calculation error. Or the variations mentioned in the second paragraph below.
What are you using to measure the miles traveled?
What are you using to calculate gas used?
How many tankfulls was used in the several hundred miles?

Every pump handle shuts off differently. These variations, combined with varying terrain on your trip, varying driving environment (traffic & roads), and perhaps even varying weather and/or wind direction, can all affect mileage.

We may have redefined top tier fuel.

Explain exactly how the calculations are being done. If it was all being done on a fill-to-fill basis, those kind of results are notoriously unreliable, especially for small fills. You need the “errors” / random variations to be spread across many miles and many gallons/multiple fill-ups. So if you really want to know, drive the next 500-1000 miles or so never letting the tank go below 3/4 or something like that. Obviously, start and end on a full tank and just keep track of all of the gasoline along the way.

Different pumps will stop at different points depending on how they are setup. This would give slightly different fuel amounts. Which would matter very little over a full tank average, but would skew the average on smaller fill up.

Something is wrong with the math here.

Something is definitely wrong, I’ve never heard of an 18 year old Oldsmobile Silhouette getting 25-27, but maybe that’s just me.

It’s not a math error. It’s a measurement error.

It’s a math error. The gas pump does the measuring…the OP does the math.

The OP is using more than one pump. They are not necessarily calibrated equally.

I wouldn’t look at pump accuracy. I’d question how accurately the OP is filling the tank each time.

I’d also look to the road conditions, wind conditions, terrain, and driving environment. Nobody gets as good a mileage driving through Chicago as they do driving through Rantoul, or driving into the wind as they do driving with the wind. There are just way too many unknown variables to attribute any accuracy to the mileage readings except as an aggregate.