[centrider]

To paraphrase Keith Olbermann, so to speak.

As usual, the most frustrating aspect of driving the '06 Prius is the differences between the calculated mpg's and the display.

Driving back from Hutchinson, Ks to Long Beach, Ca this week, the differences were awesome in two cases:

The distance from Santa Rosa, NM to Gallup, NM was 256 miles. The display showed 50.3 mpg (downhill pretty much from Santa Rosa, NM to Gallup, NM) so that was expected. I expected to fill with 4-5 gal. Shocker 7 gals giving a calculated mpg of 36! By the way, the temp was 79 degrees.

The next fill up was in Flagstaff, AZ. We're looking at an elevation gain, but still, I think the wind was either from the South (crosswind) or SE (in sailing terms, a broad reach).

Distance traveled was 197 miles (from Gallup UP to Flagstaff). The display showed 44.9. The fill up was 2.404 g. You can almost calculate this in your head. Yep. . . 82.07 mpg! Here the temp was 63 degrees.

One can expect some differences because, I guess of the bladder-like gas tank. But those two outliers. In fact, I'm inclined to assume the Gallup pumps are grossly inaccurate. So, I'll be notifying whatever department is charged with insuring pumps are accurate.

[geodosch]

But if you look at the two trips combined: 453 miles, 7.404 gallons = 48.17 MPG. The weighted average* of the MFD would be 47.8, which is pretty close to the 48.17 actual.

With the bladder tank, it's difficult to get an accurate MPG on any given tank. When I look at my MPG numbers, I always take the average over the last 4 tanks. In fact, I've always done this, because even with a 'normal' gas tank, it's not getting filled to the same level each time, especially if the amount is being rounded-off.

*You can't just average the two MPG numbers, since one is based on more miles, so must be given more weight

[centrider]

geodosch said:

But if you look at the two trips combined: 453 miles, 7.404 gallons = 48.17 MPG. The weighted average* of the MFD would be 47.8, which is pretty close to the 48.17 actual.

With the bladder tank, it's difficult to get an accurate MPG on any given tank. When I look at my MPG numbers, I always take the average over the last 4 tanks. In fact, I've always done this, because even with a 'normal' gas tank, it's not getting filled to the same level each time, especially if the amount is being rounded-off.

*You can't just average the two MPG numbers, since one is based on more miles, so must be given more weight

Ok. I got that, and even discussed it in a sardonic exchange with my grandson in which we noted the average of the two fill ups. What happens, however if we average the 82 mpg with the following 50 mpg?

Still, keep in mind that as the air pressure is reduced when elevation is gained, the size of the bladder would increase. Thus, going from around 4000 feet to over 7300 the volume of the bladder I assume would increase? How expandable is this bladder?

The volume of the fill up in Gallup, Nm. is easily explained because it works to the advantage of the station. That is not the case with the the station in Flagstaff, Az.

[geodosch]

centrider said:

What happens, however if we average the 82 mpg with the following 50 mpg?

Unless, of course, you actually did get drastically different mileage from one tank to another, eventually it has to average itself out, since there's a limit to how much extra, or less, fuel you'll put in on any given fill-up.

centrider said:

Still, keep in mind that as the air pressure is reduced when elevation is gained, the size of the bladder would increase. Thus, going from around 4000 feet to over 7300 the volume of the bladder I assume would increase? How expandable is this bladder?

The bladder is inside a tank, so it cannot expand beyond the size of the outer tank. What has the biggest impact is the temperature, since colder temps cause the bladder to stiffen, so it can't expand as rapidly.

centrider said:

The volume of the fill up in Gallup, Nm. is easily explained because it works to the advantage of the station. That is not the case with the the station in Flagstaff, Az.

I don't follow... the station has no control over how the Prius gas tank is working.

[centrider]

geodosch said:

Unless, of course, you actually did get drastically different mileage from one tank to another, eventually it has to average itself out, since there's a limit to how much extra, or less, fuel you'll put in on any given fill-up.


The bladder is inside a tank, so it cannot expand beyond the size of the outer tank. What has the biggest impact is the temperature, since colder temps cause the bladder to stiffen, so it can't expand as rapidly.


I don't follow... the station has no control over how the Prius gas tank is working.

Right. But the station (I assume) controls the volume of gas the pump delivers.

By the way, a serious question: Has your Prius ever showed mpg down in the mid 30's? This was a first for me.

[geodosch]

centrider said:

Right. But the station (I assume) controls the volume of gas the pump delivers.

The attendant can control the flow rate by where they lock-in the nozzle. The faster the flow, the more likely it will click-off before the bladder has had a chance to expand. Though I think at most stations they just try to use the fastest rate that doesn't cause it to cut-out due to gas backing up into the filler neck.

centrider said:

By the way, a serious question: Has your Prius ever showed mpg down in the mid 30's? This was a first for me.

It's funny, because my fill-up last week was over 2 gallons less that I had expected, based on my MPG & Scangauge. The attendant had wandered-off, so I tried to restart it, but it wasn't having it. So that tank calculated as 82.1 MPG. I filled-up again today, and that was 41.3, which is by far the lowest I've ever gotten, though I knew it was an artificial reading. The average of those two tanks was 59.96, which is close to the 60.4 & 61.1 MPG readings the MFD showed for those two tanks.

But my range has been between 48.1 and 65.9 (always averaging 4 tanks) since I got the car.