Inaccurate MPG?

Precisely! A lot of Prius Chatters are into an obsession with stretching tanks: risky, and completely pointless.

Yeah, found this site lately. Easy to search, ship to Canada, good resource. Anyway, there's a lot of pricey components hooked up to the gas tank, collectively item #1:

EMISSION COMPONENTS for 2016 Toyota Prius C

 

The reflected MPG was never 100% accurate for me.
The Honda Fit I now own has a similar MPG feedback.
It's similarly optimistic.

Once you figure out how Pollyanna your readings are....you can just adjust.

Since I grew up in the 70's, 80's and 90's...I'm still happy with the feedback possible.
Since when I was a kid...all you might get...was "maybe" an idea of what capacity your fuel tank was...
and then...when the gas gauge got low....you tapped the gauge and hoped the needle didn't suddenly fall all the way below E.

100% accurate? No.
Still helpful? Yes.

 

Hmm, too bad. Our 06 Civic Hybrid was always spot-on, or slightly pessimistic. As Stephen King (more accurately his character: Roland) would say, the world has moved on.

 

I don't want to be too much of a conspiracy theorist but....*now putting on tin foil hat.....
I don't think there is any legislation or laws in regards to how accurate these extra MPG readings need to be.
So I think most automakers tend to make them optimistic by design, knowing that will make most people happy...especially if they never check it manually.
You're going to fill up, before empty....so if you JUST go by the feedback and never look too hard? Well? If the ignorance makes you believe you're doing a couple MPG better than what you are actually doing?

So basically you are either ignorant....but happy.
OR...aware....and adjusting.

 

Yeah I agree. Toyota, Honda, want happy customers.

So why am I grumpy?

 

I've owned a few vehicles over the years and most have inaccurate mpg indicators. What I've heard (which may be speculation) that gas station pumps often will state higher quantity of gas than what was actually pumped into vehicle. This could mean that our in car mpg could be correct. I honestly do not care about 1 or 2 mpg when my car is already getting 47-51 mpg.

 

So I found an article about gas pump accuracy.

Keeping Gas Pumps Honest | NBC4 Washington

 

The car does not meter the gas as it's going Into the tank, just relies on rudimentary float. It does closely monitor how much gas goes through the fuel pump. So, I don't think it's the gas station that's cheating.

 

G'day,
(Idle thought-for-the-day)
Organic liquids like gasoline/petrol have a relatively high volumetric thermal expansion coefficient, greater than most (solid) metals.
Assuming the fuel is not significantly pre-warmed before being registered through/at the fuel pump, could thermal expansion explain one of the slight apparent/real differences for usage between hot and cold weather?
In hot weather, less mass of fuel is metered since its volume is increased compared to that in very cold weather, when the fuel is denser, so more mass of fuel goes through the pump under the same conditions.
I'm assuming the fuel pump being metallic (?) and within the engine compartment (?) is less affected by the external temperature while the fuel tank is more open to the ambient temperature so the fuel inside it expands or contracts a smidgen due to the season.
David S.

 

That sales fraud does happen, that is why many states or localities have inspections from AG or consumer protection departments. But it does not happen 'often', at least in my region. The 12% rate mentioned in your later link seems relatively high.

I do believe that fuel station cheating would appear the same as car MPG display 'error'. Since most of us do use many stations over the life of the car, and the great majority of stations don't cheat, we should be able to spot those that do.

 

So far, my 2012 Prius C is only 1.2 mpg off compared to my fuelly reported mpg. I'd say that's a tight margin of error on its part, considering my last vehicles in dash display would say 25.6 and my average mpg was 23.2. I'd say the Prii are pretty accurate mpg calculators all things considered.

 

As far as observed/calculated mileage vs. reported mileage, my 2014 C-One is consistent with what you're seeing (comparing your '16?)... it sounds the same, anyway (actual slightly higher than reported). As for the filling routine... I just always let it click off twice and put it away. I go the the same station most of the time (mid-commute for me - but best price), but I don't always use the same pump... so there's variation there anyway. Even if you DID use the same pump every time, there's the question if the last user dumped everything out of the recovery hose, air/ground temperature, station's actual fuel temperature (did it just come off a truck? is it early in the morning? hot day?) or whatever... stuff beyond your control. If you just get more data points you will end up with a better idea of actual vs. reported, but in my experience (I put about 38k on mine in the last nine months) I generally see my actual mileage exceeds the car's reported mileage by 2-3 mpg. Don't overthink it too much, though... it's really not THAT critical. And, beyond fueling conditions, instrumentation probably varies with every car too (even in the same model year). I work with measuring instrumentation in my job. Stuff generally isn't 100% accurate out of the box... and generally speaking, EVERY device is going to get "some" signal drift in one direction or the other over time (whether it's in use, or even sitting on a shelf).

Moral #1: Don't rely on what the car reports - it's just not going to be 100% accurate - ever. Plus it will change over time (so you're chasing your tail).
Moral #2: Don't rely on calculated mileage - there are too many external factors to account for to be 100% accurate.
Moral #3: Just accept that it's a somewhat gray area and don't run the thing out of gas because you relied on either.

 

YES! 2017 Prius 4 gives consistently that we get 63-65 mpg. Well, the calculator says 53. Dramatic after fill-up, but seemed "dead on" for the first six months. What is up with this? Is this a problem or should one just disregard the milage in the dash of a Prius?????????????

 

Yes and no. It's been discussed about a million times here already.

The "problem" is not unique to Prius. ALL makes and models with fuel mileage monitors seem to read high, with a few minor exceptions.

How much they are "off" depends on all of the same things that determines fuel mileage in the first place.

 

Can you reword: what does the car's dash display says, versus what you're calculating? If the dash display is 5~10 % higher than calculated, that's about the usual.

Toyota seems incapable of resisting "cooking the books". Difficult to justify, considering the honest numbers are the best out there.

 

I've been averaging results every 2 fillups and so far the indicated MPG is about 2% better than the amount shown by the gas pump. Haven't done enough fillups to average for 5.

I will add that I always filled my cars to the brim, my previous car went 320K+ (a 2002 Honda Accord) and never had issues. I am doing the 2 click method with the Prius.