Real MPG vs Trip computer

J

Jays07MS3

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07 Mazdaspeed3 GT Cosmic Blue
So for the past 2 months I've been keeping track of my mileage both manually (miles driven / gallons to fill up) and with the trip computer. Being an engineer, I'm very methodical about how I go about it to the point that my girlfriend thinks its silly :) But it seems that the trip computer is off by an average of +2.66 mpg on my car. I really wish there was a way to recalibrate it but until then I'll just be stuck doing the math in my head.

Anyways, I just thought some of you might be curious as to the accuracy of the trip computer. But it sure does suck to see how much money I'm spending on gas! (boom04)

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Nothing silly about that, I do the same thing. Here's the funny part, mine was like that at first where it would be off by about +2.5 but now it's off by about -.75. My car is telling my I am getting around 27.6 MPG but I am actually getting around 28.5-29 MPG.
 
Jays07MS3 said:
So for the past 2 months I've been keeping track of my mileage both manually (miles driven / gallons to fill up) and with the trip computer. .... But it sure does suck to see how much money I'm speeding on gas! (boom04)

Ouch! $3.42/gal?! That does hurt. Of course $3.10 hurts, too.
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Very interesting....

Thanks for posting this info. I'm a new MS3 owner, so I'll have to see how mine stacks up.

~Nullmod
 
richmusch said:
Nothing silly about that, I do the same thing. Here's the funny part, mine was like that at first where it would be off by about +2.5 but now it's off by about -.75. My car is telling my I am getting around 27.6 MPG but I am actually getting around 28.5-29 MPG.
Click to expand...

Wow thats strange. How many miles does your car have now? Mines a little over 10K. Did your trip computer change all of a sudden or was it a slow gradual change? In any case, I should be able to see a difference if one occurs since I'm logging every fill up. At least it makes me feel a little better that I'm not the only one with a messed up trip computer ;)
 
desperado-c said:
Jays07MS3 said:
So for the past 2 months I've been keeping track of my mileage both manually (miles driven / gallons to fill up) and with the trip computer. .... But it sure does suck to see how much money I'm speeding on gas! (boom04)
Click to expand...

Ouch! $3.42/gal?! That does hurt. Of course $3.10 hurts, too.
Click to expand...

I know! I couldn't believe it when I saw it. The prices here in socal have been insane. But this particular gas station was a bit higher than others since it was right off the freeway and I absolutely needed gas ASAP.
 
gj, jays...=D this is definitely not weird at all, because i think we all care very much about what we're actually getting for gas mileage. i haven't formally put my numbers down on a spreadsheet, but you can bet your ass that i've done the calculations out in my head every time i fill up at the pump! i'm at about 5500 miles, and i'm still getting about the same skew as you are getting (trip computer reads about 2-3 mpg more than actual). hopefully, we will both see more of what richmusch is seeing as we tick off more odometer clicks...
 
have you ever tried a tank of gas that you got on it a lot/were in boost a lot, VS one where you drove like a granny? Just a guess I think maybe the computer mpg calculator gets off when you get on it mainly and that messes up the average.
 
Hehe, I've kept all my gas receipts and written the distance travelled on the top of each one so that I can enter them into a spreadsheet at some point. My gf thinks I'm crazy/OCD too.
I think it's smart to know where your money is going, especially with gas prices today.

As mentioned above, the trip computer is maybe good for general idea and maybe not so much real accurate data.
 
I'm averagin just shy of 300 a tank, but I'm at 1600 miles. And where the hell do you all live? I'm paying $2.80-$2.85 a gallon for 93.
 
aaronc7 said:
have you ever tried a tank of gas that you got on it a lot/were in boost a lot, VS one where you drove like a granny? Just a guess I think maybe the computer mpg calculator gets off when you get on it mainly and that messes up the average.
Click to expand...

No, most of my tanks are a mix of hard/easy driving. But mostly easy. I drive 60miles on an open freeway to work 5 days/week and I set the cruise at about 80-85mph each way. But for city driving, I drive pretty hard :). I'll just keep logging my mileage and I'm sure I'll have a few tanks where I'll average <20mpg or >27 mpg. Then I'll see if the difference between the trip and calc is the same or not.

Olestra said:
Hehe, I've kept all my gas receipts and written the distance travelled on the top of each one so that I can enter them into a spreadsheet at some point. My gf thinks I'm crazy/OCD too.
I think it's smart to know where your money is going, especially with gas prices today.
Click to expand...

Thats exactly what I do. I get the receipt and write down my mileage and the trip computer reading. Then I reset both. It really irks me when the printer on the pump doesn't work. Then I have to find a scrap piece of paper to write everything down. Thats when my gf thinks its silly because she has to wait there while I find a piece of paper and document everything LOL!

Sierra117 said:
I'm averagin just shy of 300 a tank, but I'm at 1600 miles. And where the hell do you all live? I'm paying $2.80-$2.85 a gallon for 93.
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I live in San Diego and we pay that for only 91! (freak) Totally sucks!
 
I've been wondering in my mind is the reason why the MPG on the trip computer is generally inflated because when you stop your CUR MPG goes to 0, however, I know your engine is still using gas to idle.... I imagine more so if your running the A/C. I would think the computer would be more accurately if the car was always in motion, and never stopping for lights.

This is only a theory.
 
chronus377 said:
I've been wondering in my mind is the reason why the MPG on the trip computer is generally inflated because when you stop your CUR MPG goes to 0, however, I know your engine is still using gas to idle.... I imagine more so if your running the A/C. I would think the computer would be more accurately if the car was always in motion, and never stopping for lights.

This is only a theory.
Click to expand...


Thats true that at a stop your getting 0 MPG but that would also be reflected in the manual calculations. You used gas while you were just sitting there so your calculated MPG would go down as well. But whether they both go down by the same amount who knows. I'm not sure how the trip computer calculates mpg but its definitely not dividing the miles by the gallons used. Its probably based on injector duty cycle or something similar.
 
i'm too tired right now to analyze it out fully, but i think it's got something to do with the fact that it updates periodically. basically, the system takes discrete measurements at the end of each of those updates and just averages them into the total, which means there will be rounding involved and any other amount of number fudging...which they most likely round UP on. so in the end, you get an overestimate...;)
 
It's nice to see someone take the time to compare the values. My experience has been similar, approx 2 mpg difference. I usually minus 2 from the trip now.
 
I've now got about 10,700 miles on my MS3, and have experience driving it both purely highway (Boston to Wisconsin, then Wisconsin to LA) and mostly city (LA). I had seen other posts like this thread mentioning the optimistic trip computer MPG, so I have paid attention. Here's what I've observed.

During pure highway driving (1-2 stops per tank), the trip computer is dead on, within 0.2 MPG. I was getting 29.8 to 31.0 doing 75-80 mph; 340-350 miles per tank wasn't a problem. My girlfriend's Mazda 3 2.0L sedan only got 29 highway during a road trip this summer! :D Auto vs. manual perhaps?

In the city, I've been averaging just about 290-300 per tank, for an average of ~25 MPG, while my trip computer is saying 26.5-27.8. I too noticed the ~2 MPG optimism since moving to an urban area.

My theory is similar to chronus337 in that the stop and go part is what causes the difference to occur. Look again at the CUR MPG on the trip computer at a stop light. It doesn't read zero; it reads "--.--". Since it's not a number, it doesn't average it into the trip computer's average MPG. It basically ignores whenever you are at a stop. But since you are still burning gas, the calculated fuel economy will be better than actual.

Anybody want to time how long they have the engine running but aren't driving during their next tank of gas? Also, does anybody know the fuel burn rate at idle? Then we could figure out if my theory is true....

-Carl
 
Though I have 34,000 miles now, I just figured out how to see the mileage readout on my MS3 Sport. On 3 tanks I see over 2.5MPG difference, and it seems to be worse in stop & go.

However, it reinforces my observation that driving over 80MPH on long trips still knocks about 3MPG off of the average I get at around 70.
 
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