Fluctuating MPG

Is this all on the same tank of gas?

 

If this is across multiple tanks, I'd suspect variable tank fills, or difference among brands. My car definitely prefers gas from a particular station. That gas gives smoother engine performance and MPG increases although my best tank isn't as high as the lowest MPG you mentioned.

 

Wind?
You do know that hand flying the car is more economical, right?

 

o/p, i don't understand your description of the problem, can you be more specific with more detail?

 

I think you are monitoring the trait of the fuel use as you are driving and this is always constantly varying. May I suggest you try to ignore it and just see the average over an entire month, or whenever you gas up. If you reset the A Trip when you fill up, you can calculate the amount of has you've used and know how many miles you've driven. After a few months you will get a more accurate mpg that way... The car's computer is known to overestimate by up to 10%.

Don't be embarrassed, we all had to learn how things work on this Prius, but remember to come back and let us know how you got on.

 

When they put in the winter ethanol mix my gas mileage tanks. In spring and summer I get 58mpg with not much effort. I looked at my tank and I was @ 52mpg and dipped as low as 49mpg. I thought about selling my Prius I was so devastated. The feeling was similar to quitting college but family pressure to stick with it took over. I am now @ 51mpg on this tank.

You have to focus on your driving and using secret driving techniques only known by veteran prius drivers. I often refer to them as ancient Chinese secrets. The above tip about hand flying the prius is a good suggestion.

 

As you mention, initial battery charge can make a difference. How long is this daily commute?

I'm wondering - how are you measuring your MPG? If it's the dashboard display, it's only approximate, taken by computing various sensors in the car. Then again, doing a fill each day would have to be at the same bowser (pump) at the same service station.

 

Does 'same exact driving conditions' include:

same tank of gas (no change of seasonal blends or new-vs-leftover fuel mixture in tank);
same passenger & cargo weight,
same wind/light-breeze conditions (speed and direction);
perfectly dry road;
same commute direction;
same outdoor temperature profile the entire commute distance;
climate control turned off;
same engine temperature before startup (i.e. no driving outing errands during the work day, and same cooling off time and conditions since morning commute);
same battery charge level at start of commute;
same traffic light delays?

There are many MPG-relevant differences that most people don't notice, and this list isn't complete.

 

What distance is the commute, short journeys vary a lot, whereas a longer one would even out. On a 3 mile trip to the local supermarket I can get (showing on the trip) anything from 38 to 55 mpg (UK gallon) on the outward journey with a cold engine and 48 to 88 mpg on the return journey. So I am seeing variations of 38 to 88 mpg, no worries.

 

Yeah, definitely need to know the distance. Also the weather plays a big part too - I find when it is cold, the MPG is lower, but it looks like you are in Scottsdale so that shouldn't be in play here. Wind also makes a big difference. Strong headwind vs tailwind will create those kinds of discrepancies.

 

For the differences OP is talking about, winds need not even be strong. Light ones will make that difference.

This is where the our U.S.-style MPG measure is misleading or unintuitive to many people, and Euro-style Gallons/100miles measure would be more revealing. OP describes a 20% difference, but it is really (100/64 - 100/75) = 0.23 gallon/100mi between his readings. In a previous car, I easily saw a 20% difference in my cross-state travels with regular (not actual storm) winds -- 30 mpg going as a tailwind, 25 mpg coming back into headwind. That figures as (100/25 - 100/30) as 0.67 gal/100mi, triple the fuel difference that OP is describing.

 

It's nothing wrong with the car, almost guarantee it

Probably wind. And dependent on battery charge level. If it's a relatively short commute, a full battery vs empty battery could mean a significant variance in MPGs, everything else being held equal.

But I'd put my finger on wind strength and direction

I had a 10 MPH headwind driving to work yesterday, and got 54 MPG

I had a 30 MPH tailwind driving home and got 70 MPG

This morning I had a 30 MPH headwind and got 47 MPG. I'll probably get 60+ going home

 

Also:

A crosswind has nearly the the same effect as a headwind.

At 62 mph vehicle ground speed, a gentle breeze of only 10 mph can be an apparent wind speed of 72 mph or 52 mph, a difference of 38%. (edit: And since drag increases with the square of velocity, that's a difference in aerodynamic drag of 92%.)

The more efficient the drivetrain is, the greater the share of aerodynamic drag on fuel consumption is.

 

Well said.

 

For a Prius (at least for the Gen2, which was current when actual details were posted long ago), certain angles of crosswind are even significantly worse than a direct headwind.

 

That's right. If I am remembering the same, I think you're talking about Wayne Brown aka mwbueno's work at privatenrg.com :

http://privatenrg.com/indexOldSite.html#WindBigDeal
http://privatenrg.com/Crosswinds.htm#Crosswinds

A quote from the first link:

"The two Prius 'Energy Use' simulators below have been programmed with identical variables except for an 8 MPH wind in opposite directions. The Treo 650 on the left illustrates a crosswind/tailwind @ 8 MPH; while the OS6 simulator on the right is showing a wind from exactly the opposite direction. Notice what an 8 MPH wind can do to help or hinder the Prius’ MPG or fuel mileage. The Treo shows us 53.55 MPG while the screen on the right shows us 43.02 MPG --- ONLY THE WIND DIRECTION is causing the Prius to get more than a 10 MPG difference!! Interestingly, an 8 MPH wind is very difficult for us to perceive while traveling on our highways without referencing a waving flag.

Trees, grass & bushes move almost imperceptibly in these 8 MPH winds so, we often think there is no wind at all & wonder why our cars do better some days than others."​

(Bolded emphasis added by me, hint, hint, OP.)


A quote from the second link:

"Crosswinds are almost always more detrimental to mileage than a direct headwind. The first screenshot below shows a baseline with several arrows explaining much of the graphical detail. This first frame shows No Wind & the Prius getting 50.49 MPG. Next we show a 10 MPH Headwind which drops the mileage down to 43.06. The next frame shows a crosswind from 10 degrees off the right-front of the Prius and the mileage goes further down to 42.28 MPG. Follow the series of frames and watch the very many interesting effects of crosswinds. Notice also that we do not get back the equivalent mileage of a 10 MPH headwind until about 90 degrees. Then notice that the crosswinds coming even from rearward side of the vehicle do not help until they get to about 130 degrees where the 'Ambient Wind' graphics goes from Red to Green. This is called the 30/70 Wind Rule and is explained below by clicking here."​

 

Touché. My mistake, I got the wrong impression from your first post. Sorry, no offence intended.