66.7 or 55.4 ? MFD or Calculator?

Are those Imperial Gallons or have you converted that to US gallons for our sake?

Gallon

 

Did you spend a lot of time in Stealth mode?

I find that the most significant differences between the electronic calculator and the miles driven/gallons added mpg, comes when I have spent a lot of time driving in stealth mode. My commute allows for this - I have driven along at a steady 30mpg so long in stealth mode from full charge (on cruise control) that the battery goes almost to purple.

Here is what I think is happening.

In stealth mode, the electronic calculator appears to be calculating 99.99 mpg. This is going to make its calculation lower than it should be, for during stealth mode the instantaneous mpg is infinite. So, assuming that the electronic calculator is accurate when you are in ICE or battery-assisted mode, your car's calculation should be accurate if your driving pattern doesn't put you in stealth too often. But, if your pattern allows for comparatively lots of time in stealth mode, the car's computer will understate your mpg.

I have been meaning to experiment whether the car's calculator also counts time at stoplights as 0 mpg. It should not count that time at all. I am not even sure if the car's calculator is time-based, though it appears to be. If it is, and it includes time at stoplights as 0 mpg, then it is again going to understate your actual mpg.

Your mileage, of course, may vary!

 

I would think they are standard US gallons as he lives in Illinois.

 

The car's calculator merely divides miles travelled by gallons of gas consumed. It ignores the battery completely. With only a very few miles of range in electric mode, this will make no significant difference over an entire tankful, but can produce some phenominal numbers for short periods.

 

The car should calculate mpgs by dividing miles travelled by the amount of gasoline used, not considering time spent at 99.9 or 0.0 mpg.

I would trust the car computer's estimate over any traditional manual calculation. Reminds me of the time when the first calculators appeared in the mid 70s; people would use the calculator and then do the operation manually to make sure the machine did not make a mistake.

 

I'd be very surprised if the Average Consumption display integrated up the rates over time.

I expect it keeps separate totals of fuel and distance (we know it keeps distance, as it's displayed), and simply divides them to show the average.

If you do a lot of all-electric driving, it may be that you also do quite a bit of high-consumption driving when you're recharging, and the car may be less accurate at high fuel rates than at more moderate rates (driving mostly on engine power).

 

As tms says, the car merely keeps running totals of miles travelled and gas burned. And that is, after all, what your average mileage is, and over the long term (say, a tankful) it is accurate because a full battery charge is only good for a couple (?) of miles so there won't be more than that unaccounted-for in the calculation.

But over the very short term, say, a five-minute segment on the display, it can be extremely inaccurate. Sure, you may get 100 mpg for a few miles, as the battery goes from full to empty, but you'll have to burn gas eventually to recharge the battery, and then it'll show very low mpg.

On the other hand, this ability to balance load and keep the ICE running at a more-ideal speed is one of the ways the Prius achieves such high overall mpg.

jamarimutt, those early calculators often did make rounding errors, due to careless programming.

 

See, with gas pump cutoffs, "inaccurate" gas gauges, possibility of pumps being incorrect, and so on - this debate will always rage on because of too many unknowns.

You can't simply divide miles travelled by last amount of gallons filled (or current gallons filled) - you may have used more or less gallons than what you filled in the tank, simply put! On the other hand, the car display could be doing anything from measuring gas input to total miles, or judging usage off an inaccurate gauge, who knows?

If you use the 'reverse' method, of taking the computer's #'s - 55.4 MPG, and 481 miles, you get that you used 8.6 gallons of gas. It's just as likely that the pump shut off before refilling all 8.6 as it is that you used exactly 7.222 gallons.

My best suggestion is accept you'll never know "true" mileage and figure it's between the two numbers you have there. (while I still use a simple calc and keep track of that, I also keep track of the car mileage as well - just watch them both and realize neither are accurate!)

-m.

 

Oops, I got him mixed up with a poster from Great Britain.

 

I just had the exact same thing. On my first fill-up I still had 2 bars on my gas gauge showing, whereas on my second fill-up I only had 1 bar showing. Yet the numbers show that I got less gas to fill up on my second fill up than on my first, and the difference between the computed mpg and the computer is 19.

362 miles 7.9 gallons 45.4mpg 48 comp mpg
427 miles 6.5 gallons 66.7mpg 47.8 comp mpg

Both times I went to a Chevron and filled up in the morning when it was about 66F outside. The only difference was that the first time I filled up near work, at the end of my commute, whereas this morning I filled up near my house before the commute. I was really surprised when the gas shut off at 6.5 gallons. The gas gauge says the tank is full now.

Esther

 

I understand that the Prius measures the amount of gasoline delivered to cylinder 1 and multiplies it by four to determine fuel consumption. This seems much more accurate than the best one can do with the traditional method of calculating mpgs. The car does it for you... I don't see the point of doing it the old fashioned way.

 

The problem with totally trusting the Computer is that it may or may not be accurate (it may have a bias). Just like your speedometer may be a mile or two off.

With the Prius, I believe that the Computer is much more accurate than hand calclulations on a Tank by Tank basis, because the fills are so variable.

However, over many samples, the hand calculations will average out, and reflect reality most faithfully.

If you have 20K Miles Traveled and an Accurate Count of the total gallons used, the fill inaccuracies and pump variability will cancel out.

Just over 5K, my numbers are stabilizing. The Computer appears it's going to report just over 1 MPG more than actual.

Once you have determined the Computer's Bias, THEN it seems reasonable that you could use the compensated number and feel fairly confident about the actual mileage on a given tank.

 

1. Your calculation was based on wrong information. The computer is more accurate. Single tank calcualations work only if the tank is filled to the same level every time!

2. 55.4 MPG. The computer is more accurate than your incorrect calculation.

Sorry to be frank, but the computer knows more than we do. Yes, it is optimistic for most Prius as far as I have read, but it should be within 2% of the true value for most people.

I keep a lifetime average based on total miles driven divided by total gallons filled. LMPG is 48 and rising. MFD reports 48-52 lately, and individual tank calculations computed by me are almost meaningless, but tend to be 40-65 MPG, depending on how much I top off the tank after the pump stops. I usually add .5 gallons after the first shut-off.

If you really require higher accuracy, buy a fuel meter, such as those used by Consumer Reports.

-Ken

 

Man, oh, man! I guess that's why I put up with North Dakota winters! My typical commute is 10 minutes.

 

i would go with the car's calculation also as an estimated 11% of gas pumps in the US are inaccurate. your car is tracking the gas used and determining mileage based on total trip distance. the instant mpg readings play no part. in EV mode the actual mileage is nearly always much higher than 99.9 mpg anyway. there is simply not another digit available to display higher numbers.

AND

congrats on the tank!!

i have also had tanks of 56+ mpg when it was primarily freeway driving at 70 mph. the cruise control on and air conditioning at 72º.

it strikes me as funny since the EPA rates 51 highway and 60 city.

i have never gotten as LOW as 51 when on the freeway and i have never gotten as high as 52 while driving on the streets (most of the time it is as low as 45!!)

i also helped a friend deliver the Daily Olympian for 4 days to check the mileage. this was 264 stops over 37 miles. (couldnt think of a better test!!) and during this time i limited my driving of the car to just the paper routes since i have two other vehicles and still averaged 43 mpg. i wish i could have done it longer but the hours do affect my work times (i frequently start at 5 AM) so that was not possible despite the fact that my friend was VERY agreeable to me driving him around for a tank full. (we figured that would be at least 2 weeks)

so i cant tell you how pissed off i get when i read these obviously biased articles from so called car experts living in the Detroit area who claim under 40 mpg in the car. i have said it many times and i stand by the statement that i have tried to get under 40 mpg and i am beginning to think its impossible in my car except in extreme mountain conditions. i live at near sea level.

 

Jamarimutt's info about how the calculator works, based on some experiments I just did.

I tooled around an empty parking lot at about 10mph; once I was completely in stealth mode, I hit the "reset" button on the consumption screen to see how it calculated the mpg. It stayed at "0 mpg" the entire time (about 7 minutes, just over a mile) until the ICE finally had to kick in to charge the battery. The time bars showed 99.9 mpg.

Once the ICE kicked in, the Avg. MPG ever so briefly (a few seconds) read 99.9, then dropped like a rock to 60 and then into the 50's, all within maybe 30 seconds.

Later, I put the car in stealth mode at 25 mph without resetting the guage, and watched how frequently the average changed. It registered a new average about every 8-10 seconds, though I expect the calculation was distance-based.

From which I conclude....

--> The calculator won't register an mpg to begin with unless/until the ICE has run for at least part of the calculation. (Otherwise it would be infinite.)

--> However, once you have some gas used, it will take data points in stealth mode, and your average will change. I suspect it is just adding distance and not adding any gas to its ratio.

--> it has some logic built in re: how often it samples, but I can't tell what that is.

--> It can't possibly be using the gas guage as its "gas used" meter, as it registers new averages far too frequently. The amount of gas in a cylinder seems to make sense.

 

i believe the mileage is determined by the amount of gas measured in the cylinders as that is a highly precise measurement and the gas tank or gas gauge is not.

i think someone here mentioned that the readings are displayed every two seconds or so