Trip Computer MPG

How many people calculate their true MPG by hand and compare it to the trip computer?

My trip computer ALWAYS grossly overestimates the MPG I'm getting. It's usually not even close to the actual MPG.

I drove 412 miles on my last tank...
- Trip computer said 35.0 MPG.
- Actual gas mileage was 32.5 MPG. It overstated the fuel economy by 2.5 MPG!

That has been the most egregious overstatement, though it is almost always off by 1.5 - 2.0 MPG.

My guess is the Mazda engineers (correctly) assume most owners do not check their mileage by hand.
So they can program the trip computer to state wildly optimistic MPG numbers, and consumers will be none the wiser.
 

7eregrine

The man, the myth, the legend
:
Land of Cleve
:
2016.5 CX5
This gets asked about every 3 months.
I tracked mine for 3 months and it was always within 1.
 
:
2018 CX-5 GT AWD + Tech
I've been tracking fuel economy since I got my car about a year ago, and I'm with bluegrass on this one. My manual calculation is always lower than stated by the on board computer. Not a big concern for me, since it is within my personal acceptable range.
 
:
Denmark
:
CX-3 Optimum Aut.
Mine isn't a CX-5 but a CX-3 though I think the computer is pretty much the same.
My actual mpg is usually (about 95% of the time) within 0.5 MPG US, rest is of by 0.5-1.5.
Sometimes the actual mpg is better than the computer, sometimes worse.
Mind you I've had to convert from L/100km to MPG US, but that shouldn't change anything.

So no, I don't think Mazda has programmed their computers to be wildy optimistic.
 
:
2016.5 CX-5 GT AWD titanium/black 2016 Miata Club ST MT white
I always track my fuel economy in all my vehicles. I have a lil notebook in each one and it all gets entered, along with my maintenance notes.

That said, I never double check the stated economy to the actual economy. What's the point? It's just for reference. What would I do if the two numbers didn't agree? That's right, nothing.
 
I always track my fuel economy in all my vehicles. I have a lil notebook in each one and it all gets entered, along with my maintenance notes.

That said, I never double check the stated economy to the actual economy. What's the point? It's just for reference. What would I do if the two numbers didn't agree? That's right, nothing.
If the trip computer was actually accurate, you wouldn't need to calculate it manually every time.

And it would provide a realistic MPG reading for the 98% of people who don't calculate manually.
 

7eregrine

The man, the myth, the legend
:
Land of Cleve
:
2016.5 CX5
A lot of people calculate manually because it is so ridiculously easy nowadays.
This app:
https://play.google.com/store/apps/details?id=com.kajda.fuelio

has been installed over million times. And there are many others.
And here's one for the Apple heads:
https://apps.apple.com/us/app/fuelly-mpg-service-tracker/id295905460

I always track my fuel economy in all my vehicles. I have a lil notebook in each one and it all gets entered, along with my maintenance notes.
1995 called bro and wants you to get an app. Some of them synch to your PC or let you download it all if you want. Notebooks?!?
;)
 

Kaps

Contributor
:
CX-5 Touring 2016.5
If the trip computer was actually accurate, you wouldn't need to calculate it manually every time.

And it would provide a realistic MPG reading for the 98% of people who don't calculate manually.
The car calculates the mpg based on certain measurable factors but it never calculates based on actuals. It never calculates the volume of fuel used and uses that it approximates it based on some sort of a formula.
what this does - some people get the same mpg as their car gets. Some get less and some get more. When I was keeping note - my real world was .5 to 1.25 mpg more than what my car showed - and this was due to my commute cycle and my drive style. I tend to coast a lot. It depends on your driving style / commute and other factors.

This inaccuracy isn't so much of an issue for me - it seems that different CX-5s have different thresholds for the fuel light. I have never filled more than 11.5 - 11.8 gallons and I think that is just too conservative. I would love my CX5 to let me drive and use up 12.5 gallons at the very least. Atleast drive 350 miles before needing a fuel station.
 

yrwei52

2016 Mazda CX-5 GT AWD w/Tech Pkg
Contributor
:
Plano, Texas, USA
I always track my fuel economy in all my vehicles. I have a lil notebook in each one and it all gets entered, along with my maintenance notes.

That said, I never double check the stated economy to the actual economy. What's the point? It's just for reference. What would I do if the two numbers didn't agree? That's right, nothing.
Like you I have a little notebook and have been written down the same information in it.

Ive checked the computer calculated MPG only few times so far, and the difference is within 1 MPG most of time but computer calculated MPG is always higher.
 

ColoradoDriver

Gen-1 Kodo Design
Contributor
:
Denver, CO
:
2014 CX-5 Touring
I've never tracked my MPG on this car in the 6 1/2 years I've owned it. I look at the dash tracker every now and then and all I know is that it's a lot better than the barely 15 MPG I was lucky to get on my last car!
 
what this does - some people get the same mpg as their car gets. Some get less and some get more. When I was keeping note - my real world was .5 to 1.25 mpg more than what my car showed - and this was due to my commute cycle and my drive style. I tend to coast a lot. It depends on your driving style / commute and other factors.
You're the first person I've heard from whose trip computer underestimates MPG.

There was an article in Car & Driver that showed discrepancies between trip computer vs. actual MPG, across many brands/types of cars.
In every case, the trip computer always indicated better MPG than the vehicles actually got.

Regardless of my commute or driving style (in town driving, aggressive rush hour commute, steady highway cruising, ect), my trip computer ALWAYS reports considerably higher MPG than actual.
 

shadonoz

SkyActiv Member
Contributor
:
State of Jefferson
:
2017 CX-5 GT AWD+
The car calculates the mpg based on certain measurable factors but it never calculates based on actuals. It never calculates the volume of fuel used and uses that it approximates it based on some sort of a formula.
How do you know that? Can you tell us more about this method or where to learn more about it? I'm trying to imagine how it would work without measuring the fuel.
 
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2018 AWD GT Premium Red/Black
I've been tracking fuel economy since I got my car about a year ago, and I'm with bluegrass on this one. My manual calculation is always lower than stated by the on board computer. Not a big concern for me, since it is within my personal acceptable range.
Me too, usually about 1 MPG, although I stopped checking most of the time months ago and just accept the mean shift.

I usually fill 13+ gallons, I start thinking about getting gas when the light comes on unless I have a long trip planned.
 

Avoidin Deer

Zoom Zoom, baby
Contributor
:
Central Virginia
:
2019 CX-5 Reserve
I've been keeping mine in a spreadsheet every time I fill up so I can track mileage by octane & brand. I have about 4,400 miles on my car and am on my 17th tank of gas.

Any variances between the computer and manual calculations even out the next tank or two of gas.

It only takes a little under-filling the tank when you start off to make it look like your mileage declined (because it took that much more gas to fill up at the end of that tank). Conversely, if you under-fill some at the end of the tank, it looks like your mileage is higher than the computer stated because it seems you went through a lower amount of fuel.

It's not hard to do. Not all gas stations are perfectly level.

But (surprisingly) the computer is accurate.
 

Kaps

Contributor
:
CX-5 Touring 2016.5
How do you know that? Can you tell us more about this method or where to learn more about it? I'm trying to imagine how it would work without measuring the fuel.
I don't know the specifics - it could use some sensors to verify how long the throttle position is depressed and how quickly it is depressed to calculate approx. fuel consumption and match it against distance but I don't think any car measures the volume of fuel used and takes that into calculation. That is true for all makes and models. So this is again a guesstimate and it depends on driving / commute etc. There are circumstances where it overestimates fuel used and others where it does not.

Main point is Mazda or any other maker is not trying to fool anyone by showing higher than actual mpg on the dash. It is a calculation and is not accurate in all conditions.
 

Pitter

Pitter
Contributor
:
2020 CX-5 Signature Azul Metalico
I have no reason to monitor the miles per gallon (or kilometers per liter which is what my car reports) because I climb a 14 kilometer mountain road almost every day and know full well my milage is going to be lousy. What most interests me is the reported range I have left since I am sometimes in areas without gas stations.
 
Any variances between the computer and manual calculations even out the next tank or two of gas.

It only takes a little under-filling the tank when you start off to make it look like your mileage declined (because it took that much more gas to fill up at the end of that tank). Conversely, if you under-fill some at the end of the tank, it looks like your mileage is higher than the computer stated because it seems you went through a lower amount of fuel.
What you said would be true if the trip computer were accurately estimating MPG.
And this is why I know mine is NOT accurately doing so...

For ALL of my 50+ fill-ups, the trip computer has shown higher than actual MPG. It NEVER evens out.
I can stop pumping at the first click, or fill it to the brim after 3-4 clicks.
The trip computer will always show an artificially high MPG.
 
:
2018 AWD GT Premium Red/Black
I've been keeping mine in a spreadsheet every time I fill up so I can track mileage by octane & brand. I have about 4,400 miles on my car and am on my 17th tank of gas.

Any variances between the computer and manual calculations even out the next tank or two of gas.

It only takes a little under-filling the tank when you start off to make it look like your mileage declined (because it took that much more gas to fill up at the end of that tank). Conversely, if you under-fill some at the end of the tank, it looks like your mileage is higher than the computer stated because it seems you went through a lower amount of fuel.

It's not hard to do. Not all gas stations are perfectly level.

But (surprisingly) the computer is accurate.
Yes, but that under-filled tank will have yielded a higher calculated mileage for the previous driving. It should work itself out if you do it each time. If this were the source of the discrepancy, the Mazda should compute better mileage sometimes, and worse others. Mine consistently reports better from the computer than odometer change/fill volume.
 

shadonoz

SkyActiv Member
Contributor
:
State of Jefferson
:
2017 CX-5 GT AWD+
I've been keeping mine in a spreadsheet every time I fill up so I can track mileage by octane & brand. I have about 4,400 miles on my car and am on my 17th tank of gas.

Any variances between the computer and manual calculations even out the next tank or two of gas.

It only takes a little under-filling the tank when you start off to make it look like your mileage declined (because it took that much more gas to fill up at the end of that tank). Conversely, if you under-fill some at the end of the tank, it looks like your mileage is higher than the computer stated because it seems you went through a lower amount of fuel.

It's not hard to do. Not all gas stations are perfectly level.

But (surprisingly) the computer is accurate.
Yes, but that under-filled tank will have yielded a higher calculated mileage for the previous driving. It should work itself out if you do it each time. If this were the source of the discrepancy, the Mazda should compute better mileage sometimes, and worse others. Mine consistently reports better from the computer than odometer change/fill volume.
What you said would be true if the trip computer were accurately estimating MPG.
And this is why I know mine is NOT accurately doing so...

For ALL of my 50+ fill-ups, the trip computer has shown higher than actual MPG. It NEVER evens out.
I can stop pumping at the first click, or fill it to the brim after 3-4 clicks.
The trip computer will always show an artificially high MPG.
Well that settles it then, doesn't it.
Avoiding Deer's works the way it's theoretically supposed to. And he's verified it.
Murky's and bluegrass' don't work that way, and they've verified it.
That's life!
 
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