Actual MPG much lower than system generated MPG readout

[sushd]

Since last 5 fuel-ups, the actual MPG has been much lower than the system generated MPG figures, about 10MPG lower!! 

Started with a fuel-up where the tank took in 47.5 litres to full-up. Suspecting it was a faulty pump at the station, the next fuel-up was done at a different pump but at the same station. This fill-up to full was less than 45 litres, as usually is, but the actual MPG was again 10 below the system readout. Since then I have had 3 more fuel-ups, all at different stations, but the problem remains! 

System MPG          Actual MPG

61.1                           52.2

59.2                           48.4

58.2                           49.2

 

Earlier the difference between the two used to be about 2-4 MPG (system readout > my calculations).

One thing I have observed on the display is that the hybrid Battery discharges pretty quickly as compared to before. And that the engine starts up as soon as I put my foot on the accelerator in "D" mode, even if the hybrid Battery is almost fully charged (green bars, and not blue). It seems like the car doesn't drive itself on Battery that often as it used to before.

Could this be an after effect of the HHC? Was done first time ever last year ...

 

Used to get high 40s / low 50s when the car had the original 12V battery but this was replaced last year and the MPG had jumped to low 60s. Since installing the new 12V I now also charge the battery using CTEK every now and then. And, have checked the 12V battery and that it seems fine (14.2V, 12V).

 

Not sure what the problem could be!

[FROSTYBALLS]

3 minutes ago, sushd said:

System MPG          Actual MPG

61.1

So, are we meant to guess what the actual mpg was ....

[sushd]

Apologies, inadvertent mouse clicks posted the topic before I could finish writing it.

[Joseph D]

Are you trying to squeeze every last drop in, or are you stopping when the pump clicks off?

[sushd]

I continue doing exactly the same as I used to before, nothing has changed about how I fill up a tank to full.

That said, would a squeeze to the last drop result in a difference of 100 miles per tank full? Even at its best performance to date, my car would need approx. "7 litre" squeeze every time! 

[Joseph D]

13 hours ago, sushd said:

have checked the 12V battery and that it seems fine (14.74V, 12V).

As an aside as I don't think this is the reason for the change in MPG, but presumably this reading is when the car is in "READY" mode. If so this tells me the car thinks the Battery is flat.  If you have somewhere that is secure, e.g. a garage, you really need to pop the bonnet when you park up for the night and then measure the Battery voltage in the morning at the jump posts in the engine bay. Do this before opening any doors etc. Let us know what reading you get.

[sushd]

I used the system diagnostics to check the Battery. The 12V reading is shown when in "accessory" mode (single click of power button, foot off the brake pedal) and the 14.2V reading is when the car is in "ready" mode. The old (original) Battery, that I replaced with the current one last year, used to show 10V in "accessory" mode.

Unfortunately, can't test the Battery as you suggest as I don't have a multi-meter! Will see if any of the neighbors have one that I can borrow for a while.

[Sooty]

Have you "Zeroed" the MPG meter on the display after every fill up?

If you haven't, then the original over read will carry over to the next ones. I hope I have explained that well.

I zero it off every fill up and only get about 2 - 3 mpg difference.

Also if the first pump was faulty, your calculations will be way out compared to the car. I can't imagine how you managed to get 47.5 litres in the tank that is a 45 litre one. Unless you were sucking fumes from the tank before you filled up.

[PeteB]

The Gen 2 Prius had a larger tank (as did the Gen 1) and was very dependant on the slope of the forecourt when filling up as to how much one could squeeze in - I often filled at a site where the forecourt sloped significantly towards to front offside wheel and could get an extra 2 gallons in!

My record fill was 52.49 litres (11.55 UK Gallons) - which would take me some 200 miles before the first bar on the fuel gauge disappeared.

My record on the current Gen 3 is 40.29 litres.

I note the Gen 4 claims to have a tank smaller by 2 litres than the Gen 3.

[sushd]

On 4/11/2016 at 8:53 AM, Sooty said:

Have you "Zeroed" the MPG meter on the display after every fill up?

If you haven't, then the original over read will carry over to the next ones. I hope I have explained that well.

I zero it off every fill up and only get about 2 - 3 mpg difference.

The MPG readout and the "miles" readout on the MFD automatically reset to zero (on their own) every time I fill up. Never had to use the reset button (is it just my prius??).

Also, I always reset one of the two trip meters - that I use to calculate MPG (manually) - every time I fill up.

I photograph / note down the system MPG readout, miles done on system MFD, and the trip meter, before I open the fuel cover lid. Then I fill up the tank (always to full). And finally check both MPG & trip meters are set to zero before I put the car in "D" before driving off. Old habit!!

[sushd]

On 4/10/2016 at 3:20 AM, Joseph D said:

If you have somewhere that is secure, e.g. a garage, you really need to pop the bonnet when you park up for the night and then measure the battery voltage in the morning at the jump posts in the engine bay. Do this before opening any doors etc. Let us know what reading you get.

Having rested the car for 24 hours, locked but with bonnet open, the multimeter shows 12.44V at the jump posts in the engine bay. 

The multimeter shows 12.29V at the Battery terminal (keyfob used only to unlock doors to access 12V battery).

[Joseph D]

16 hours ago, sushd said:

Having rested the car for 24 hours, locked but with bonnet open, the multimeter shows 12.44V at the jump posts in the engine bay. 

The multimeter shows 12.29V at the battery terminal (keyfob only used to unlock doors to access 12V battery).

Well, that doesn't seem bad at all, not fully charged, but a long way from flat.  Interesting you get 14.74 V in READY mode.

All the best getting to the bottom of the change in MPG.

[FROSTYBALLS]

My multi information display has become somewhat unreliable. Start of March the average fuel consumption displayed dropped from 34-37 around town to 25-28 for a couple of weeks. Has recently been showing between 30-34.

I keep the display on average consumption. Sometimes it will change of it's own accord to cruise range.

Yesterday morning I filled up (30 litres) and zeroed the average consumption. For the rest of the day average fuel consumption varied between 47 and 68mpg, and is currently showing 51. Bit unrepresentative for journeys in and around Birmingham, where I usually get 35-37 indicated.

Actual fuel consumption based on brim-to-brim figures is in the 30's in and around Birmingham, depending on temperature, traffic, etc.

 

[sushd]

Okay, let me put the issue in a different way!

After replacing the 12V Battery last year, my "actual" MPG had jumped from between 45-50MPG to 60-62MPG. These actual MPG figures are about 2-4MPG lower than the MPG readout that the system generated - so inline with expectations. 

But since the last 5 fuel-ups, my "actual" MPG has fallen to 48-52MPG whereas the system generated MPG readout is about 10MPG higher everytime, in the range 58-62MPG. 

That means for every tank-full I "actually" drive approx. 100 - 120 miles less than before as a matter of fact (the real problem and issue here) - irrespective of whether the system generated MPG readout is accurate or not. 

 

Now, 

1. Assuming the system generated MPG readout is accurate  - what is causing the 10MPG reduction in actual and why is the system is not capturing the true picture(e.g. - leakage in fuel tank or somewhere in the line but before the point the system uses to measure MPG, system fault that has resulted in more fuel consumption but this is not being recorded by the system)? 

 

2. Assuming the system generated MPG readout is inaccurate then there are two issues:

a) what has caused a sudden fall in "actual" MPG from 58-62MPG to 48-52MPG all of a sudden?, and

b) what has caused the system to report MPG figures inaccurately at the same time (never had inaccurate MPG readouts before)?

 

 

I feared that the 12V had gone flat because the MPG figures were just a little bit higher than what I used to get with the old Battery but given the voltage reading mentioned above the 12V Battery doesn't seem to be cause.

Such a huge difference in MPG can't be because of under / over inflated tyres but that is also the cause as I check the tyre pressure on a regular basis.

Neither the driver has changed nor the way the driver drives.

So what can it be ... ?

 

 

 

Maybe I should have titled the topic differently .... but what I've stated above is the real issue I am facing.

 

 

 

 

 

[YarisHybrid2016]

I'd be interested to know exactly how fuel consumption is metered. Is it actual fuel flow, or a calculation based on ECU demand?

With the actual suddenly being significantly below the reported, I'd get it looked at by Toyota. Whilst the metering system isn't exact, to get a sudden divergence suggests a problem.

Are you aware of whether the ICE is running more often/longer than before?