Accurate Average MPG display

M

MikeM.

Member
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2013 Mazda CX-5 Touring AWD 2.0L
The trip computer on the CX-5 is the first trip computer I've ever used that accurately calculated the Average MPG figure when compared to the figure derived from manual calculations. I know this to be true because I have carefully tracked it over 1600 miles of mixed driving. All my previous vehicles were wildly optimistic when it came to Average MPG (1.2 MPG to 2.1 MPG optimistic). My F-150 truck reports 2 MPG high compared to manual calcs (and that's a lot when it's only reporting 19 MPG).

On the other hand, I see some owners reporting their trip computers are around 1 MPG too optimistic. I wonder why there is this variation.
 
I think part of it has to do with the GT speedometer reading a little off. Calculations I've read estimate ~1.8-2% off.
Edit: Due to the 19" Wheel/Tire package
 
Mine reads 32.9 average; however, I've been getting around 31.5 calculating it manually (Fuelly). Sport model with 17s.
 
Hmm, well that rules out the 19" theory. Maybe there is an impact from altitude or a factory calibration issue? Mine reads consistently off as well (.5-1mpg optimistic vs fuelly) but I have the 19s
 
Fuelly is not too accurate either. I wrote this in their forums as well

For people who use trip miles instead of their odometer. Say your last full tank of gas got you 400 miles. What happens when you put this info into Fuelly is that they take the 400 miles you got out your last tank and divide it by the number of gallons you just filled up. So in fact they are calculating that you drove 400 miles on what you just filled up, even if you have 3-4 bars of gas left. Within time the average of all fuel-ups will be close to your own calculations but trying to do it on a tank to tank basis vs fuelly will give you headaches :)
 
Gio26NJ said:
Fuelly is not too accurate either. I wrote this in their forums as well

For people who use trip miles instead of their odometer. Say your last full tank of gas got you 400 miles. What happens when you put this info into Fuelly is that they take the 400 miles you got out your last tank and divide it by the number of gallons you just filled up. So in fact they are calculating that you drove 400 miles on what you just filled up, even if you have 3-4 bars of gas left. Within time the average of all fuel-ups will be close to your own calculations but trying to do it on a tank to tank basis vs fuelly will give you headaches :)
Click to expand...

Huh?

If your trip computer was reset at the last fill up, and you divide the number of miles shown by the gas you just put in... the numbers should be plenty accurate... There is also a mechanism in place for a partial fill up which doesn't count the MPG until the next FULL fill up.
 
Your trip computer gives you the miles you just drove on the gas that you just replaced by filling up the tank... Yes, the shut offs are different but if you fill up at the same place or top of the tank each time it should be pretty consistent. The inaccuracy is not in the very simple formula that they use, but in our filling habits.

(Trip computer indicating miles driven since last fill up) / (Fuel put in to replace what was used to drive those miles) = MPG

Fuelly is always giving you the number for the previous tank, the current tank is impossible to measure outside the car. Calculating miles your are about to get would be very dependent on how you drive while using that tank... If you go from a highway trip to stop and go or lead foot then it could be very different.
 
This thread is making my head hurt. Truthfully, my entire soul is aching.

C'mon Gio26NJ, MPG calculations are not rocket science. Please pause and think before you commit your thoughts to writing.

Humans are widely believed to have intelligent thought. Please do not single-handedly attempt to disprove this theory.
 
Nothing personal Gio but the clown is the one who doesn't think the amount of fuel burned the last tank is represented by the amount of fuel to fill the tank full once again!

And as far as the Nav system, I don't want it to be exactly like a Garmin in look and feel but I do wish it displayed essential navigational information such as which way is north! And that is just the beginning of a long list of basic things the Tom Tom cannot display and cannot be customized to display.

I've been using navigational devices including Loran's from the 1970's, radar units from WWII and early GPS devices from the 1990's and I've never seen a navigational device that was so lacking in basic functionality. Hello, this is 2012! Even the most basic charting Loran from the 1970's had the ability to display a north arrow. I can't help it if you happen to love the Tom Tom software or only use it to navigate between home and work, the fact that I require more functionality doesn't make me a clown.

To add insult to injury, the deficiencies of the Tom Tom could be remedied with better software - it would not increase the price of the hardware at all. We are already paying for the GPS receiver, large and excellent screen, etc. Too bad the functionality of all that is crippled by poorly conceived and executed software designed by people who apparently have never traveled outside their own hometown.
 
Of all the times, I filled up over lunch and Fuelly EXACTLY matched my dashboard! (best mileage yet as well).
 
You guys are taking it really seriously :) I am simply happy to be above 32mpg!! Well yes, plus/minus slight variations when I calculated manually but still so much better than the next small SUV! When I tell my friends they are all jealous how good the mileage is. :):)
 
lol and bfd.

My average MPG readout has not been reset since day 1. It's 4% optimistic, I had hoped it would be more accurate than that. I maintain a spreadsheet to track every fillup for the last 9K+ miles.
 
CX-SV said:
My average MPG readout has not been reset since day 1. It's 4% optimistic, I had hoped it would be more accurate than that.
Click to expand...

I suspect the 19" wheels/tires that come on the GT model are slightly different diameter than the 17" on the Sport and Touring Editions. Have you checked your odometer calibration over a 5 or ten mile course? If your odometer is reading low, your actual MPG may be better than your calculations indicate.
 
this post needs a " like" button.

MikeM. said:
This thread is making my head hurt. Truthfully, my entire soul is aching.

C'mon Gio26NJ, MPG calculations are not rocket science. Please pause and think before you commit your thoughts to writing.

Humans are widely believed to have intelligent thought. Please do not single-handedly attempt to disprove this theory.
Click to expand...
 
MikeM. said:
I suspect the 19" wheels/tires that come on the GT model are slightly different diameter than the 17" on the Sport and Touring Editions. Have you checked your odometer calibration over a 5 or ten mile course? If your odometer is reading low, your actual MPG may be better than your calculations indicate.
Click to expand...

Yes, worth checking even if unlikely, speedo would read low too I think.
 
CX-SV said:
Yes, worth checking even if unlikely, speedo would read low too I think.
Click to expand...

My experience has been that speedometers are often calibrated differently than the odometer/trip meter even when both instruments use the same input. My CX-5 with 17" wheels and new Yokohama tires has a speedo that is spot on but the odometer reads about 1% too low. As the tires wear the odometer will become more accurate while the speedometer will read too fast.
 
Gio26NJ said:
Oh really? (braindead so anyone who has a different way of thinking and factors in other things to disprove a theory is automatically unintelligent? and you're good with that?
Click to expand...

I don't really care if you think in or out of the box. I found his post comical.
 
MikeM. said:
My experience has been that speedometers are often calibrated differently than the odometer/trip meter even when both instruments use the same input. My CX-5 with 17" wheels and new Yokohama tires has a speedo that is spot on but the odometer reads about 1% too low. As the tires wear the odometer will become more accurate while the speedometer will read too fast.
Click to expand...

Yes, different calibration for speedo and odometer might make sense in the case of CX-5 based on my experience with it.
 
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