Difference between displayed and actual MPG

Wow.
Awesome data.
7 yrs, 90K mile, 178 fill ups.

If I understand correctly, the conclusion:
Dash display if 5.58% higher than actual.
Actual miles vs dash is 3.0 mpg lower.

Just awesome.
Thank you.

 

My error, pretty much from day one, is 7.21%

 

This is great. I have not kept track of the difference but I always noticed a difference between what I calculated and what was displayed.

I figured that the gas pump makes a difference, and there was some error in the software.

I typically see 3-4 mpg difference.

The difference is low when I have idled a lot. Maybe fuel usage measurements have an error at higher flow.

 

If Toyota was a butcher they'd have a thumb on the scale.

 

Well here I am on my very first full-tank-under-my-control on a new to me 2014 Prius V and I got exactly what you are seeing: 5.9% discrepancy, 46.9mpg (calculated by Prius computer) vs 44.3mpg (calculated by me) on a 370 mile tank using 8.33 gallons of regular on 35PSI tires. I want to add more air next.
Not impressed with Toyota for never apparently seeing fit to tweak their algorithms by, say, 5% to bring these two into closer alignment. But impressed with you Vman455 for doing such excellent work for the benefit of this group. Thank you!

 

The discrepancy likely is by design.

 

Well I’m sure it isn’t accidental, or unknown, but what exactly do you mean by design?

 

The car knows more accurately, but adds a 5~9% “improvement”.

 

Calculated, and actually, are always different. Every manufacture does this.
99% of the cars will not get what they say it will get. They use a test track. They drive
slower and more carefully to get the maximum amount.
I don't know if they include wind resistant or not. Or drafting. It's likely an "average" of
speed driving and amount of fuel being used(calculated) to come up with an average.

My real mpg is 1-1.5 lower MOST of the time. For me, close enough is good enough.

 

From reports here they seem to have curtailed the mpg exaggeration with 5th Gen Prius (2023+).

 

This is the part that I find baffling.
Toyota knows its customers. We (fuel efficiency nerds) are a recognized demographic among its Prius customers. We know how to calculate real world fuel economy. And of course notice (and are peeved by) this discrepancy. So why not throw us a bone, fix the discrepancy, own the matter?
As for the rest of its customers who we may imagine care less about fuel economy, may not themselves calculate it manually, well lying to them, making as if their fuel economy were 6% higher than it is serves no purpose that I can understand, except to increase cynicism, undermine their integrity as a player in this world.

 

That is very interesting, though not clear to me why it would take them most of 25 years to fix that error.

 

I think you are talking about something else. Maybe the EPA test cycle?
This topic is about an observed and consistent discrepancy between the car’s fuel economy computer that calculates a mpg figure based on your/our driving that is as folks here have confirmed off by about 6% from what we can all easily calculate by hand by dividing fuel purchased at the gas station and miles driven.

 

epa was not mentioned. I am typing about the difference between calculated mpg
and actual mpg. I have no idea how you came to that decision since I did mention fuel.

And this is NOT a discrepancy or an error. I have no idea why you would think that.
There is a +/- for that "guess". Filling up at different pumps will give you different fuel amounts.
Each handle is different. You would have to fill up at the same pump at the same time of day
under the exact same temps and humidity to get an exact reading.
There are too many variables.
It's not a big deal either. I never trust what the computer says I'm getting.
I always calculate it myself.

Several times I get more mpg than what the display says. But the next time I fill, it's a lot lower.
Which tells me that handle cut off too soon. Or the other too late.

You can likely adjust that reading if you have techstream or similar gear.
Same with the speedometer. My actual mph is 1mph lower than what the speedometer says.

 

you didn’t mention EPA but you mentioned test track and wind resistance and manufacturer.
If you read the first post in this topic the masterful work he did to generate those data already deal with the variability you just mentioned.
We are at this point most certainly talking past each other.

 

A lot of us do track actual mpg. I use my trip-A for fill-ups, and trip-B for maintenance. A lot of the variations are related to your ability to fill the tank up to the same point in the gas filler neck. That's at least a gallon of variation.
This also doesn't take into account the odometer/speedometer is usually fast by 2-3 mph based on GPS data; but we work with the data that's available.

LoL, you need to find another butcher.

@9watts, Toyota sells millions of cars a year, so a hundred complaints don't even register. Regulatory is focused on safety, so if the gauges are within a statistical normal, it gets a pass. That doesn't even take into account that it makes them look better, in the eyes of the EPA. I remember my statics teacher telling me that, There are liars, damm liars, and then there are statisticians.

Enjoy....

 

Do you mean if the error falls on either side of accurate? I've been tracking mpg virtually since a few months after getting our 2010 (in 2010), both with a spreadsheet and Fuely, and the dash display has always been off, anywhere from maybe 3 to 9 percent, and guess which side. I would blame the variation in error on pumping vagaries.

In my spreadsheet I note the displayed mpg (well kms liters per 100 km), calc the discrepancy tank-by-tank, and the average error. Which comes in around 7%.

You may want to proofread that.

 

Yes,, I set my cruise control @ 70 mph across the flat valley floor and my GPS is bouncing between 67mph & 68 mph. It is repeatable on a straight line for about 10 miles. Hilly sections with twist and turns would introduce more variations = less reliable data.