What Drives Us

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-Tony Schaefer

Many times, when discussing hypermiling techniques, someone will ask about how to approach a hill, or what to do in rush-hour traffic, or when driving in bad weather can’t be avoided. The only sane answer is, “get over it.” Let’s face it: if you are going to scrutinize every single mile or every single minute, you will go insane. That is why hypermilers talk in terms of tank averages and lifetime averages.

Tank Average

This should be your smallest unit of measurement except for the rare exception. It is only over the course of an entire tank that you can take multiple factors into consideration: morning versus evening commutes, good weather versus storms, etc.

If you wish to maintain daily logs – and for various reasons, I have suggested just that – bundle them into the tank average and then throw them away. Daily logs are for analysis and review only. For example, another article suggests keeping daily logs for the sake of identifying the best daily commute. Once these logs have served their purpose, dispose of them.

The easiest way to calculate the tank average is to divide the miles driven by the amount of gasoline used to refill the tank. However, here are some factors to consider:

• Some cars use bladders inside the gas tank to help contain vapors. Depending on the ambient temperature, the bladders might be more or less flexible in warmer and colder conditions, respectively.   For example, the second generation Prius used a gas bladder. In the summer, almost 10 gallons of gasoline could be pumped whereas in the winter as little as 8 gallons was the maximum. The Prius fuel bladder was removed starting in 2010. This is an issue because it adds a variable when attempting to accurately calculate the amount of gasoline used during the tank.
• There is always the debate whether the on-board calculations provided by the car are accurate enough to be used. Some people choose to perform their own calculation rather than trusting the car. Anecdotally, some people have used both and shown that over a period of time, the over/under evens out and both methods arrive at the same Lifetime Average. However, for the individual tanks, which method you use is up to you.

Lifetime Average

Of course, the granddaddy of all averages is the overall Lifetime Average. This reflects your entire driving experience with the car. Lifetime averages do not need to start when the car is brand new; it is the one-number record of you and the car working together as a team, regardless of how old the car was when you two first met.

As you can imagine, calculating the lifetime average requires that you know exactly how many miles you have driven and exactly how much gasoline you have consumed. Not just for one tank or one month or even just one year. In order to calculate an accurate lifetime average, you must have been recording accurate fuel data for the entire time you have been driving the car. Trust me: this can get tedious but it is what must be done to achieve the goal.

12-Month Rolling Average

As you drive your car year-over-year, you might become curious whether you are becoming a better hypermiler. Actually, it’s great to constantly want that feedback to spur improvement. The problem is that the Tank Averages can’t be compared one-to-one and after a while the Lifetime Average barely budges.

This is where the 12-Month Rolling Average comes in. Whereas the Lifetime Average will forever be influenced by those first few crappy tanks, the Rolling Average will eventually let them go to reflect how you’re doing now. Though you can’t throw them out completely because they are part of your historical record, they are no longer an accurate representation of your current driving ability.

Calculating the 12-Month Rolling Average requires that you go back one year (sounds pretty obvious, doesn’t it?). Where it can get complicated is that the number of tanks will vary. For example, if you average a refill every other week, you cannot simply take the last 26 tanks and assume that’s a year. There will be those long-distance road trips in which you consumed a full tank in only two days. So just be careful and make sure to do the math based on the dates: sometimes 28 tanks, sometimes 32.

Conclusion

If you only keep two averages, they should be the Tank Average and the Lifetime Average. These give you a real-time feel for your hypermiling abilities and an overall view of your entire driving experience. Adding the 12-Month Rolling Average provides an updated perspective, showing how you’ve done over the past year.

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