Gas mileage

I just completed a 4 day 2300 mile trip from New London Ct., to Milwaukee, WI., and return. I drove my 2001 Chevy S10 PU with a 2.5lt 5 speed manual transmission. The trip was made in two segments. New London to Erie, PA 540 miles, then Erie to Milwaukee, 540 miles following the same in reverse. On the first three legs of the trip the truck was empty except for two small suite casses and a cedar chest. I averaged 21mpg running at 68mph. On the last leg Erie to New London I had a 1916 Olds rear end and radiator along with a case of soda the same to suite cases and a couple more pounds. I was running right around 70mph the entire day but this time I got 21.66mpg. Why would having more weight and driving harder increase my gas milage when all else was the same?

gas milage increased to 22.66 not 21.66

Any statistician will tell you that your two values are within the range of statistical error. That is, the determination of fuel economy requires a large set of values for accuracy. There are simply too many variables. A single run can easily produce a figure that is 1-2 mpg above or below the statistical norm for no apparent reason.

Statistically, your mpg was the same for both runs.

As SteveF said, the figures are well under the threshold for statistical significance, which means it could very well just be chance, or error somewhere. It could be, too, that going you were driving into the wind, and coming you had the wind at your back. That could explain the difference.

Agree; gas mileage has too many variables; the most obvious is where you gas up and how fast the pumps clicks off on you. Headwinds, tailwinds, traffic density, pavement condition, etc. all affect your gas mileage. I have been driving back and forth between 2 cities 200 miles apart for the last year, and NEVER got thee same round trip mileage with the same car. The best you can do is to keep track with every fillup and see if a trend develops. Plot the results on a piece of graph paper. Gas mileage is like the stock market, daily changes mean little; it is the trend that matters!

It’s downhill from Erie to New London. Over 600 feet! And that’s on the water in Erie.

Yep, most likely elevation but statistically insignificant. Back in the 60’s I’d make the 200 mile trip to school in Sioux Falls and would always get better mileage one way than the other. Only thing I ever figured out was the change in elevation.

Another factor could be any weather changes since variation in temps, humidity, and barometric pressure could affect it.
The amount of difference here is pretty hard to gauge.

A 1916 Olds, huh? Guess NAPA won’t have too many listings for parts on that one! A museum here in Oklahoma has a 1903 model Olds and it’s amazing that any of these cars or parts have survived the years at all.

Your method of measuring didn’t account for different fuel pumps from which you dispensed gas. There are too many variables for you to know for sure just from recording miles traveled and fuel used to fill the tank. Also, your odometer only records tenths of a mile. It isn’t precise enough for to give you two decimal places in the results of your calculation.

The fuel used is usually measured in thousandths (to three decimal places), but the miles traveled is only measured in tenths. Therefore, your calculation is only precise to whole numbers. Unless you know that you traveled precisely 540.015 miles, for example, you only really know that you got about 21 miles per gallon in both circumstances. The least precise measurement used in the calculation limits the precision of your answer.