Mpg's and Computer Accuracy

[SubCompactor]

How accurate or reliable is the mpg computer ? I fueled up today and the computer read 32.3 mpg. I then checked the accuracy by dividing miles driven by the gallons to fill tank which yielded only 29.67mpg. That's about 2.6 mpg off. I then calculated (using the computer's avg. miles per gallon figure of 32.3) to see what the amount of fuel in gallons I would of had to be to get the 32.3 mpg and it was about .744 gallons less than what I actually put in. (And yes, I do try to fill the tank up)

So this perplexes me and I am left wondering if these computer's are not figuring on FULL fillups OR they are flat out inaccurate or what? Wondering if someone could shed some light on this subject so I can either feel good about the computer reading or the old fashion method that I used to check the computer with today. Thanks for your input!

 

[PGTBrow]

I'm pretty sure that average is based on the entire life of the 2 so far. Mine has slowly started to decrease by a tenth every week... damn right foot...

 

[arcxer]

You know you can reset the average by holding the INFO button for couple seconds?

 

[SubCompactor]

I'm pretty sure that average is based on the entire life of the 2 so far. Mine has slowly started to decrease by a tenth every week... damn right foot...

ehhhh.... I don't know about that. It kinda seems that after refueling it kinda resets itself because it knows that the tank is full again thereby the miles till empty is posted. At any rate, should the computer be ignored and just calculate manually like I did?

 

[SubCompactor]

ehhhh.... I don't know about that.

I totally misspoke here. PGTBrow your right that Avg consumption keeps up with the car's overall average unless it is reset per owner's manual. It is really disheartening that some of my mileage figures per tankful are around 2 mpg less. My current fillup was my 6th and have a little over 1300 total miles. I sure hope that I will see 32mpg's in city stop and go traffic like I thought I was getting from looking at that darn computer when it was just showing overall average. Geez.

 

[SubCompactor]

....... in other words, I hope to see improved mileage as the car gets some more miles on it.

 

[DrWebster]

Mine seems to be about 1mpg too high. I reset it every time I get gas. Considering it's normally putting me as averaging 39mpg (and I usually get 38mpg when I compute it at the gas pump), I'm not gonna complain.

 

[SubCompactor]

Mine seems to be about 1mpg too high. I reset it every time I get gas. Considering it's normally putting me as averaging 39mpg (and I usually get 38mpg when I compute it at the gas pump), I'm not gonna complain.

38 mpg. Wow! Do you ever do any city driving?

 

[genericmoniker]

I'm averaging about 32 MPG 90% city. Computer shows I'm getting something close to 34.

 

[SubCompactor]

I'm averaging about 32 MPG 90% city. Computer shows I'm getting something close to 34.

This is what I am thinking. Hopefully most owners are reviewing their gas receipts and dividing miles driven by miles to fill tank and not letting that computer reading fool them. Until I "RESET" the INFO button the consumer avg was 32.3 (about 98% stop and go CITY). Comparing that figure with my actual receipts, the best so far was 32.6 mpg. ALL the other tankfuls fell just short of 30 mpg. To AVERAGE 32.3, I would of had to have some 34+mpg tankfuls to average in. And again, I have the 5-speed and am really trying to get Good mileage. Maybe it just takes time and having more miles on the engine to get 32ish mpg's in the city but I know for sure to not trust that Consumer Avg.

 

[DrWebster]

38 mpg. Wow! Do you ever do any city driving?

Maybe 10% city, 90% freeway...though a good 25% of my time on the freeway is stop-and-go.

I've found that 55mph is the 2's sweet spot for MPG. I try to never go faster than 55 during my commute, and even with the stop-and-go I manage 38mpg. On a road trip to Nebraska last summer I was getting about 34-35mpg, but was going 65-70mph.

 

[DrWebster]

I should also note that I keep a small notebook in the car that's specifically for recording actual gas mileage. Every time I stop for gas, I note the date, number of gallons put in, and how many miles I went (I use the trip odometer and reset it each time I get gas). I have logged every tank of gas I've put in the car since I drove off the lot, not just for my 2, but also for my wife's 3 (which used to be my DD). I'll eventually plot it out all on a graph to see if there's a long-term trend.

 

[MazdaTwo]

I should also note that I keep a small notebook in the car that's specifically for recording actual gas mileage. Every time I stop for gas, I note the date, number of gallons put in, and how many miles I went (I use the trip odometer and reset it each time I get gas). I have logged every tank of gas I've put in the car since I drove off the lot, not just for my 2, but also for my wife's 3 (which used to be my DD). I'll eventually plot it out all on a graph to see if there's a long-term trend.

Sounds like a Fuelly.com account would save you a lot of trouble, sir.

 

[SubCompactor]

Sounds like a Fuelly.com account would save you a lot of trouble, sir.

Say MazdaTwo, I would like your take on my observation(s) with the computer and ALSO I looked at your awesome Fuelly account with your heavy highyway usage. I wanted to know since I am primarily driving city if I should be getting better mileage after things loosen up (only have 1400 miles). I ask that since I notice that you mileage figures seems to have been consistent right out of the box. Thanks!

 

[MazdaTwo]

Say MazdaTwo, I would like your take on my observation(s) with the computer and ALSO I looked at your awesome Fuelly account with your heavy highyway usage. I wanted to know since I am primarily driving city if I should be getting better mileage after things loosen up (only have 1400 miles). I ask that since I notice that you mileage figures seems to have been consistent right out of the box. Thanks!

I've definitely seen an improvement in MPGs over time. If you go about 3/4 of the way down my Fuelly page and click on the "view annual reports" link, you can see that my average over the first 3000 miles was 33.7mpg; but over the following 25000 miles that increased to an average of 35mpg in 2011. You'll notice that my average so far in 2012 is like 36.5mpg; I attribute that to two things: 1) I've started using Shell gas exclusively (although I still use 85 Octane (10% ethanol) I find that the quality of gas is better and my mileage shows it), and 2) My wife bought me an UltraGauge for Christmas. Since it shows real time MPG, I can immediately see how my speed/aggressive driving is having an impact on my fuel efficiency and can make the proper adjustments.

As for the computer, I noticed that my calculated mileage rarely lives up to the computer estimation. I have the sport model, so I don't have the built-in trip MPG; However the UltraGauge has both Trip MPG and Short Trip MPG (resets every time the car is started) and the estimated fuel economy is always a little better than what I actually end up with. Some days are closer than others. I filled up this morning and my MPG was 37.3... the computer said it should have been 37.9.

Finally (sorry for being long-winded), It's really hard to estimate the percentage of "highway" compared to "city" in my area. I definitely don't live in a city; however, the percentage of miles that I spend on the actual interstate is like maybe 20%. My area is a heavily populated suburban area with mass amounts of traffic lights... so it's a lot of getting up to about 55 for 2 or 3 miles and then coming to a dead stop. Rinse. Repeat. It's not highway, but it certainly isn't city. So take that for what it is. I'd say I'm really closer to 50/50 highway/city.

 

[PGTBrow]

So yea... I drove to DC and back this weekend and my avg went from 32.9 to 33.0 so I guess thats good lol...

 

[SubCompactor]

And that was going to be my next question. What would you say that the "sweet spot" is for getting better and best fuel economy? Like maybe 4th gear for sustained stretches at 40+mph? And then better 5th gear for right around 55? In other words, what should be my targets knowing that I do drive a lot of city but access the highways at times. (highway speed limits set at 70)

 

[DrWebster]

Sounds like a Fuelly.com account would save you a lot of trouble, sir.

I'll probably sign up for one eventually, but part of me still likes having it all in the notebook too.

 

[MazdaTwo]

And that was going to be my next question. What would you say that the "sweet spot" is for getting better and best fuel economy? Like maybe 4th gear for sustained stretches at 40+mph? And then better 5th gear for right around 55? In other words, what should be my targets knowing that I do drive a lot of city but access the highways at times. (highway speed limits set at 70)

The Jury is still out on that for me. I almost always get better mileage in 5th gear all the way down to 35mph (as long as it's pretty flat, obviously) but yeah, you get similar mileage in 4th gear at 40mph. As far as the highway goes, 55-65mph in 5th gear is the sweet spot. This car doesn't exactly love highway speeds over 70.

 

[genericmoniker]

Starts to feel kind of scary after that point anyways haha.