[evaunit6]

This one's a little bit harder, I think:

Now I'm only on my 3rd tank of gas since buying my Prius, but I've noticed a huge disparity in what my fuel gauge and the consumption screen say in regards to miles per gallon. According to my consumption screen, I'm averaging just under 47 miles per gallon, yet in just over 200 miles, I've lost 6 bars of my fuel gauge. I'd say around every 37 miles I lose a bar...something's not adding up.

Maybe I didn't fill the tank all the way; I didn't know anything about the whole rubber bladder thing. I just pulled the pump out when it clicked itself off. Could this be the case or is my Prius screwed?

 

[JKnight]

I'd like to know about this, too. I filled up, took a trip, filled up again and figured 30 mpg. The gauge told me 47+ mpg.

Are the gauges programmed by Diebold, by any chance?

Seriously, I just did the "click off" thing, too, at two different gas stations. Is there that much leeway in when one pump clicks off vs. another one?

Jan

 

[DanMan32]

At an average of 55 MPG, the first 6 bars (except the first) I get about 30 miles. The last 4 bars about 50. First bar I get anywhere from 180-220 depending how "full" I filled the tank.
I have gotten 87 miles on the blinking pip before running out of gas. I once got 87 without running out of gas. Guess I was about to run out on that one.

All gas gages are not linear. It's just that we expect a linear guage since it is digitally displayed as pips/blips/blocks.

Some of us are firm believers of not topping off. I am not one of them. I usually put in a gallon or so of gas from the shutoff.
Because of the tight seal of the Prius tank, even around the pump nozzle, the pump might click off prematurely. More often though, the premature kickoff occurs when you START filling, not as much when ending.

 

[Astrowoman]

This probably should be a FAQ... actually at least 2 of them. First of all, the fuel guage is nonlinear -- so are most analog gauges, but for some reason people expect the pseudo-digital "pip" system to be more so. One pip is fewer miles at the empty end than at the full end.

The screen uses the fuel injector to calculate MPG. This will always be consistent and should provide really good data about relative performance for YOUR car (last tank to current tank) but could have an overall bias if your particular fuel injector performance differs much from the norm.

Finally a caution -- when you use lifetime or long baseline average perfomance to manually calculate your MPG for comparison, be very careful about how you do the calculation. When you calculate on a per-fillup basis you use the number of gallons that you just put in because you assume that was how much you just used up... depending on how you do the calculation over a longer baseline you may need to leave it out.

In either case, many people report that the "hand" calculation is more favorable than the car's calculation. I choose to rely on the car's calculation since I really care more about current performance relative to my overall norm than I do about lifetime average performance compared to other Prius driver's. (See, bear, I have made SOME progress with my addiction!)

 

[john1701a]

Newbies typically fixate on the gas gauge. Fortunately, most tend to let go of that 20th Century habit after awhile. Too much happens at the pump to take the "full" level too literally, especially when the seasons change.

The Multi-Display is a better choice for judging performance. And even though it does round optimistically, the number balances out to a surprisingly consistent value.

1.4 MPG is what I've been seeing for a difference.

Just subtracting that from the MPG on the Multi-Display is all I do to to figure out what I'm truly getting.

 

[Rmarchena]

Forget about it...

My friend you will never know your Prius's MPG. You will be guessing or expeculating about it.
The Prius designers acomodated all the readers-indicators in such a way of variables that is impossible to know an accurate number of MPG.
The Prius's MPG up today is secret mistery that everybody knows and resists to accept in the real world of the streets.
Please, do yourself a favor and forget about it.
Instead, measure the driving time that get per tank and you will amazing results that you can compare against the Prius itself in different readings and against other normal cars.
The famous bladder is the key of the inconsistencies for all the excuses as per a lot of posts in this forum. Do not loose your time finding out.

 

[efusco]

I think you got your answer, but it's a bit lost, IMO, in the morass of posts.

To summarize:
On any given tank the computer displayed mpg is much more likely to be reliable than is the calculated. That is b/c it is measured from the fuel injector and will be very consistant (note I did not say "accurate"). As John pointed out it tends to be a bit optimistic. I find around a 2 mpg discrepancy in the summer. In winter I find it to be nearly exactly on target with a tendancy toward better calculated mpg. I'd say it's nearly a wash overall.

Over time the lack of accuracy with the computer will begin to show an accumulate. But with the hand calculated method even though the tank to tank averages will be less consistant, over time it will be the most accurate with the only variables that contribute error being a)measurement of distance traveled and b)accuracy of the various pumps used in displaying the amount of fuel pumped.

Finally, the answer to your question, I think, is "yes". More than likely you underfilled that tank a little. You'll probably find that your calculated mileage for this tank will be less than the computer mileage. But then the next tank calculated will be more than the computer showed as you make up for the prior underfill...it all comes out in the end.

The one good point rmarch makes is, don't sweat it. Give yourself about 3 or 4 months of driving, keep track of the mileage during that time and then reassess then...I'm confident you'll be happy that both methods of determining mileage are accurate as I described above.

 

[Old n Bold]

Yes, JKnight, there is variation in the shutoff sensitivity from gas pump to gas pump. Where I usually purchase gas, one of the pumps clicked off after one gallon. I fought it all the way to 9 gallons then quit. It took very little back pressure to cut off. Don't use that pump anymore.

With my previous Explorer, on a trip, I clicked the pump on the last click (fastest refueling) and proceeded to clean the bugs off the windows. Hearing a bad sound, I came around the SUV to find the tank full and fuel gushing onto the ground, under the car. Must have overflowed at least a gallon and the pump never clicked off.

Because of the small sampling of the 11+ gallon tank in the Prius, it doesn't take much difference in fueling to make a large varience. So to gain a larger sample and cancel out variance in fueling, I log every gal. of fuel into the tank to the nearest tenth. Using the entire amount of fuel used to divide into the total number of miles driven makes any variation in individual tank consumption insignificant. For 6,000 miles, my Prius has used 112 gal. or 51 mpg. I would say that this is statistically accurate.

For curiosity, I wondered how accurate the MFD reading on mileage could be. Since there is no convenient way of getting the MDF mpg reading for the life of the car, I simply averaged the readings for a close estimate. The average was 49.7 mpg or about 1 mpg less than calculated. Pretty close.

The mileage has improved this summer. Probably due to warmer days, engine breaking in, and improvement in my driving habits brought about by the car. The current tank indicates 54.4 mpg on the MFD so I expect the lifetime to creap up a hair, but as I am getting more total mileage on the car, it will take great shifts to cause significant changes in the lifetime mpg.

In any event, it is doing a lot better than Consumer's Reports reported. They got 41 mpg in combined city/highway. I have never seen a tank get that low.

 

[OverTork]

I'm a "non-topper" myself so I get very inconsistant fills. This causes an average discrepancy per tank beween indicated and calculated MPG of 3.44 MPG. It has been as high as 7.9 MPG and as low as .7 MPG. However it's not always + or - so after 2600 miles and six trips to the pump the lifetime calculated and the lifetime indicated MPG are only .22 MPG different (indicated being slightly above calculated at this point). Six fills is not a large sample group but on my car it seems the indicated milage is probably pretty accurate. I attribute the discrepancy to inconsistant fills.

 

[DanMan32]

Another possible reason the MFD might be optomistic is evaporative emissions control. The engine sucks vapors from the tank for a bit, and puts it in the intake manifold, after the injectors, thus bypassing the fuel usage measurement. Ever notice your power and MPG is a bit better just after fueling and at the end of the tank?

As an analogy, consider MFD MPG calculations to Ded Reconing and hand calculations to GPS. GPS will accurately show your possition, within 300 feet. Ded Reconing will measure within feet for short distances, but over time you could be miles off course.

Averaging MFD readings is fine, as long as each entry is over the same number of gallons. But you could easily throw it all off if you had short tanks with consitently great, or consistently poor MPG. This could cause a weighted average.
Better to calculate number of gallons MFD says you used (miles/MPG), add the gallons up, then divide that sum with the sum of the miles.

 

[BIF]

Or just use the GreenHybrid database.

That's what I do. It makes things easier.

 

[DanMan32]

Have they fixed it so you could retroactively enter your previous tank fills, and that it doesn't confuse last year's recordings and put them ahead of this year's?

 

[ehurtley]

I have a 0.28% discrepancy over 20888 miles. (MFD calculation shows 45.91 mpg, gas pumped shows 45.61 mpg.)

 

[Penni]

I have a similar problem, and if I am understanding this post correctly, I think it has been very helpful for me to stop by here.

I have had my prius for almost 6 months, and after playing all kinds of games with my meter, I finally decided to start tracking a whole tank on my screen and comparing that to the manual calculation at fill up. What I've discovered over the last 10 or so tanks is bizarre. The screen is remarkably consistent-usually around 48-49 mpg, unless I'm on some unusual trip. However, the manual calculation has a distinct pattern. Every other tank is very low (about 41-43mpg) and the other tanks are high (51 or so). I was thinking that I should trust the manual calculation over my computer, but after reading this, it's interesting to learn that the computer is probably more a better gauge of performance. And I can tell you it's a BIG relief because I couldn't understand how I could possibly get such widely disparate mileage driving the same commute with the same driving behavior (i.e. coasting at all the same times, certain rates of acceleration at certain places, etc....all things I have strategized on over the months of experimentation).

I actually am about to go in for my 10,000 mile (I'm overdue at almost 11,000 miles...never thought I could put this many miles on a car so fast!) and I was going to ask the mechanics to look into it, but I have a feeling that the expertise here is far greater than anything I could get from the random guy at the Toyota Service Center.

From what I'm reading, what I think is far better is to track the computed mileage to gauge performance and periodically track the calculated mielage over total miles and total gallons (as opposed to averaging each fill up) for a more accurate picture of how I'm doing.

Maybe I'll try topping off, too, to try to get more accurate fill-up calculations.

 

[jshore]

For curiosity, I wondered how accurate the MFD reading on mileage could be. Since there is no convenient way of getting the MDF mpg reading for the life of the car, I simply averaged the readings for a close estimate. The average was 49.7 mpg or about 1 mpg less than calculated. Pretty close.

This should be close enough, at least if all your fill-ups are a similar number of gallons and/or your MFD readings don't vary that much. The more accurate way to do it would be to use the MFD reading and the number of miles after the fillup to determine the number of gallons the car's computer believes that you used. You can then add those gallons up over several fillups (or the full life of the car) and divide the total miles driven by that amount to get a more accurate measure of the mpg according to the MFD.

Doing the above computation, I have also found that the discrepancy between MFD and MPG calculated from what the gas pump says is ~1 MPG over the life of the car (47.5 mpg by MFD vs. 46.4 mpg by direct calculation).

In any event, it is doing a lot better than Consumer's Reports reported. They got 41 mpg in combined city/highway. I have never seen a tank get that low.

Ah...The joys of a temperate climate like Bellingham's! Here in Rochester, I had a tank of gas that corresponded almost perfectly with the 2 most brutally cold weeks here...and I got only 35.6 mpg on the MFD on that tank! It probably doesn't help that my commute is only about 15 minutes or so, so the car never really got to warm up very well. I do get similar mileage to you in summer though. (My current MFD reading at ~250 miles is 53.4 mpg.)