another calculated vs. actual MPG question

You're first mistake is assuming you had X number of gallons in the tank. The GenII doesn't work that way.

Do a search for "bladder" and you'll learn more about the GenII Prius fuel system.

 

Sounds like a big difference to me. Bad gas? or maybe the meter on the pump was off?
Or you didn't fill it up all the way the 1st time?

Have you ever gotten that little mileage before?

 

Now you know why it's called the "Guess Gauge". It's usually high but sometimes low. I got a more realistic estimate by using a 10 gallon tank (1 gallon per pip) and the actual miles driven. If you get a tank with a high average don't celebrate too much until the next is in the book. One high & one low are just two average tanks with a fueling problem.

Since one never knows how much gas is in the bladder conservative wisdom is:

At 2 pips start looking for gas.
At 1 pip buy gas.
At the flashing pip buy gas NOW.

 

Sounds pretty routine to me

I have an '09

Tank before last I got an all time record 89.7 mpg (2.86 gallons for 256 miles) - which is insane.....I was topping up at the end of September because I keep a monthly record. My dash display had me at 65.5 mpg for that (half) tank. That's what - 30% off?

But the tank just wouldn't fill beyond 2.86 gallons - and sure enough, the first pip fell at just 56 miles the next tank, meaning it hadn't filled completely.

You have to keep a running record of the fillup volumes/miles traveled. Even if you always fill a the same pump and always fill to the automatic cutoff, it will vary.

I find that the average over 4-8 fillups ends up routinely about 3% less than the dash display (i.e. the dash was 2 mpg "optimistic" over the past 9 months - giving me 61 mpg when I only got 59 mpg based on calculation from miles driven/pumped gas).

Actually, even in summer, when filling at the flashing last pip, I've never got more than 10 gallons into that tank.

 

Since before I bought my 2010 prius I've had phones with GPS on them. I know that my speedometer is roughly 2 mph generous (haha cops). But my mpg display is consistantly 2.1 mpg generous. I think that's a wash, but math was never my long suite.
I'll be happy for other explanations.
Tony Renier
Green Bay, WI, USA

 

I've been keeping track of miles and gallons on my 2005 since I got it in 2004. I do a 10-fill-up rolling average in a spreadsheet and there's still a lot of week-to-week variation in the graph. You just can't second-guess the bladder.

 

My record isn't as long (just passed 1 year and 36k miles), but I find that the 8 fillup smoothing yields a consistent result. For example, between May 19th and October 9th, my rolling "smoothed" MPG number has ranged only from 63.2 MPG to 66.1 MPG (i.e within 5%) ..........while the dash readout has varied a good deal more (59.9 to 71.8, or 15%).

You could say, therefore, that 8 fillup smoothing is "too much" to account for the bladder, and is wiping out natural variations due to weather, speed, conditions (and whether the wife drives or not).....at least in my case.