consumption screen question

There will be other posters that can better answer you question. Welcome to PC. You're question is a common one for new Prius owners, and along with some of the other questions, brings something to my attention.
Many of us old-timers that have tracked our MPG over the years may assume that everyone has the same understanding of what exactly MPG is and what it does and does not represent or imply. It's a term everyone knows and uses but it's actual usefulness and purpose may be misconstrued on occasion.

So I thought that I would make an attempt at trying to clarify the uses of the term. Please feel free to critisize, add to or ignore the post entirely.

O.K. MPG 101

Raw definition - Miles(traveled) Per Gallons(used to travel those miles) Or (Miles driven) divIded by (Gallons used).

Meaningful definition - How many miles did I average for each gallon of gas used; or how much gas do I use to go a certain number of miles. MPG is a ratio, a number, that represents your fuel efficiency under different circumstances.

MPG can vary from 0(zer0) when idling at a light, to infinity(99.9) on you MFD, when coaqsting down an incline and the ICE(internal combustion engine) is not using fuel.

MPG is only meaningful when associated with a time interval, such as;
MPG per Tank
MPG on my Vacation
MPG on my city commute.
MPG if I try hypermiling.
MPG w different fuels, tires tire pressures, drivers (I weigh over 100lbs more than my wife), etc.
Lifetime MPG for vehicle

Next, in order for comparasions of MPG to be meaningful between cars or drivers or even the same car and driver but different circumstances, there must be something in common. For example, probably the most common is MPG per tank. But even this can be misleading unless the average is tracked over 10 fill-ups or more due to the bladder(another topic). And it is really only meaningful to you since it is doubtful that any other member on PC has exactly the same set of driving conditions and citcumstances that you have.

While MPG comparasions between cars, drivers, fuels, conditions, etc, may be misleading unless conducted under extremely controlled conditions, there is a wealth of knowledge to be learned from it's relative changes when seemingly only a single condition is changed. Such as, increaseing tire pressure and my average MPG per tank seems to be better. And if that works for one PC member, it maY work for others. If I, here in metro Atlanta, switch brands or grades of fuel and experience an improvement in MPG. you may not get the same result in Boulder. Here, most all brands come though a common pipeline. I assume Boulders comes by tanker. Different sources.

This post is in no way meant to dis the hypermilers, I have done it myself myself once in awhile over the years. Almost killed myself and family comes down of Lookout Mountain in TN, and forgot the Fiesta had a locking sterring wheel. What an idiot!

The points that I am trying to make are:

1. MPG can vary widely. Some of which is under your control and some is not.

2. MPG needs to be associated with a time frame to be meaningful. And keeping records for different time frames are also independently meaningful in trying different things to reduce your fuel consumption. I am averaging X MPG per tank. I got X MPG on our vacation. I get X MPG in city driving. I get better MPG on this route.

3. Tracking MPG is mostly relative only to you, your car, the fuel you use and your particular driving conditions. While comparing the changes in your MPG under different conditions and circumstances is meaningful when shared between members, the actual MPG that one or another member may experience may not be relative to you.

4. Finally, unless you are or want to be a hypermiler, relax and enjoy your car. Keep a log if you want to be able to compare changes in your driving habits, such as learning to "pulse and glide", or other changes that may affect your MPG. I personally plan to log reset at each fillup. You need to do what work for you. The Prius does a real good job on it own, is what I understand. Sit back and let it do the work for you.

 

Evan's response summarized the situation nicely ... and concisely! One thing I have reverted to since the year I have owned my 2008 Prius is to make a habit of recording the MPG and miles driven since the last fill-up, as shown on the Consumption screen. Dividing the miles driven by the MPG will give you the gallons of fuel consumed since the last fuel stop (assuming the screen was reset). So you will know how much fuel, at a minimum, you should be able to pump aboard. That may or may not result in the tank being filled to capacity, depending on the amount pumped at the prior fuel stop.
Keeping a running log of all your cumulative fuel consumption is also useful for computing lifetime fuel efficiency, relative to total vehicle mileage.

 

2 times now I have not hit reset and yet the mileage number dropped all the way back to O on it's own. It seemed to happen around 342 miles. But the MPG did not seem to reset, (it did not change as wildly as it does when you actually hit reset - due to being based on less miles). Is there a limit to the maximum number of miles this displays?

 

Certainly not as low as 342 miles!
However, I have come across another way the MPG resets without touching it, that is if you use Scangauge device and its Fillup function. This seems to reset my MFD screen (so I have to remember to write the numbers down beforehand!)

 

Miles reset when you refuel. MPG requires a manual reset by pushing the screen button.

Tom

 

Natural followup to that question is "how do I make use/sense of that reading?" Obviously the more regenerated the better. But what is 50Wh with respect to the battery capacity? Is that i.e. equivalent to 10% of the SoC? What should I look for for optimal efficiency? High MPG with low regen? Low MPG with high regen? Or is that reading just there for pure curiosity?

 

You want high MPGs with as little regen as possible. The less you use the traction battery, the better, because using it unavoidably incurs losses.

The ancient wisdom of maximum fuel economy still holds: "drive as though you had no brakes". Of course these days that tends to maximize road rage, so it must be applied judiciously.

 

I am glad I read this! I always thought that at a fill up that miles and MPG both reset automatically when I filled my tank. I had an 04 and recently traded to an 08.

 

Yes, the behavior has changed since your 04.

Tom