• Welcome to Mazdas247, the 24/7 Mazda Community and Resource. Learn about the benefits of becoming a Member here.
  • We're an Amazon Associate and eBay Partner. We earn from qualifying purchases.

Real Bad Gas Mileage!

CXRabbit

Proud New CX-7 Owner!
:
2009 CX-9 TR +Kenwood DNX7020EX
Alpha, that's a good description and assessment, and so far in my experience, there HAS been a fuel efficiency hit. Since I was able to do more highway miles on my last tank of gas, I got a better feel for it as compared to before I had the update done, and my MPG have definitely been worse.

Problem is, it's also been real cold here too, so I can't really say how MUCH of an efficiency hit there is.
 
:
2019 CX-9 GT Machine Grey
CXRabbit said:
Alpha, that's a good description and assessment, and so far in my experience, there HAS been a fuel efficiency hit. Since I was able to do more highway miles on my last tank of gas, I got a better feel for it as compared to before I had the update done, and my MPG have definitely been worse.

Problem is, it's also been real cold here too, so I can't really say how MUCH of an efficiency hit there is.
Here is an interesting websight on the outside effects on gas mileage. Particularly the section on cold weather. This may help to answer why Mileage has been poor. Its been 5 degress the last 2 days in Boston. Some things that many dont consider includes the fact that if you have a short commute your mileage will be significantly worse. My wife only goes about 7 miles to work. By the time she gets there the car is probably at optimum operating temp for about 5% of her drive.(especially in freezing temps).
I have also seen many complain that they are doing 70MPH all highway so they should be getting great Mileage.. The article points out that doing 70MPH vs 55Mph will reduce you MPG by at least 12%
Lots more info at the site.

http://www.artsautomotive.com/Mileage.htm
 
Last edited:

CXRabbit

Proud New CX-7 Owner!
:
2009 CX-9 TR +Kenwood DNX7020EX
I've read all that and do understand that cold and driving habit effect the mileage, and sometimes in big ways.

But I know for me, I'm referring to the DIFFERENCE within the last few weeks, while my driving distance and style remain the same. The only change has been the cold weather and the update. According to that article, cold weather could account for 5.3% decrease in efficiency. If that were all I was experiencing I'd be losing less than 1 MPG in nearly identical driving conditions/distance. The difference I've seen is more like 2-4 MPG.

Not discounting you or other possibilities at all... I realize cold weather may be partially to blame... it's just that I've seen a pretty drastic drop-off in mileage... but it's hard to know for sure what's causing it.
 

9Hooker

Aviator Extraordinaire
:
AWD CX-7
from the above link:
1) Fuel atomizes poorly when cold. The fuel must be atomized to be burned in the combustion chamber. If a smaller percentage of the fuel atomizes, more fuel must be added to provide enough atomized fuel to burn. Therefore all cars run a "rich" mixture when they are cold, the colder the engine, the richer the mixture. The engine will eventually warm to operating temperature (about 190 deg. F), but the colder the engine is, the longer it will take. Even after the engine is fully warmed up, cold ambient air will continue to cool as it mixes with the fuel, causing poor atomization.
I don't buy into this really. There's no hard data behind it. I would be more inclined to believe if there were some numbers... like the nitrogen tire pressure thread.

190 deg F (generally on most cars) is the temperature at which the thermostat opens letting coolant that has absorbed heat from the cylinder walls and head out of those places and into the radiator, and coolant from the radiator into the motor to absorb more heat. Cylinder wall/head temperatures are considerably higher, factoring in things like specific heats and the temperature differential of the coolant in the radiator. Why 190? Well water boils at 212 (210??), and even with coolant and system pressurization the boiling point is not going up too high. It could be considerably higher. General aviation uses air (and oil) cooling for their motors, and they run about twice that temperature.
 
:
'04 RX-8 || '10 CX-9
what kind of numbers are you looking for? the premise behind that paragraph is sound. it's not purporting to calculate what effect a given drop in temperature is going to create. the fuel is cold all the way through the injector, regardless of cylinder wall temp.
 

erhayes

Contributor
:
Mazda CX-5 FWD Touring
Has anyone reported on their gas mileage under the following conditions? Engine broken in for 5,000+ miles, tires at recomended pressure, relative flat country and a 200+ mile highway trip using cruise at a given speed like 65 or 70 or 75 mph? If one is on the turbo frequently then I expect the gas millage to be down. WOW bad run on sentence But hay it is that kind of day. LOL
 
Last edited:

azcat

US129 Dragon Slayer
erhayes said:
Has anyone reported on their gas mileage under the following conditions? Engine broken in for 5,000+ miles, tires at recomended pressure, relative flat country and a 200+ mile highway trip using cruise at a given speed like 65 or 70 or 75 mph? If one is on the turbo frequently then I expect the gas millage to be down. WOW bad run on sentence But hay it is that kind of day. LOL
Well, yeah, somewhere I did; I thought it was in this thread. Under those conditions (5k+ miles, relatively flat, cruise on 74, no hard acceleration), I got 23 mpg, and I've got AWD. I haven't had the "smooth" ECU mod. I think, with the same conditions except the cruise set on 65 or less, one could get the "estimated" 24 mpg, provided one didn't get a semi in one's tailgate. However, since I couldn't use the same pump at the same station, the "long fill/short fill" thing might have skewed the results.
 

9Hooker

Aviator Extraordinaire
:
AWD CX-7
dmitrik4 said:
t's not purporting to calculate what effect a given drop in temperature is going to create.
Not trying to be argumentative here, but in the article he does claim that 50% of potential gas mileage can be recouped/lost by some means or other. I don't put any stock in that. Well maybe some speculative stock, but no more than 20% of my portfolio.

His statement is "Fuel atomizes poorly when cold". Can that be proven? I'm sure it can but I'm too lazy to do the google search and the author doesn't give any evidence to back it up. Until someone can back it up one way or another I will believe his premise is false.

Talk about what you know right...

So then airplanes (surprise I went there, right?) have fuel tanks in their wings right... It's really REALLY cold at altitude. The airflow over the wings FURTHER cools the fuel to below ambient (fuel from internal tanks is recirculated to the wing tanks to further cool it). Why would they do this if it atomizes more poorer (sic).

There is not the same energy potential in fuel regardless of temperature. As temperature goes UP, fuel expands. For any given volume of fuel, there is more energy contained the colder the fuel is.

Then there is the issue of viscosity of fuel vs temperature... but if the pumps and injectors are doing their jobs it should be a non issue.

And this does not take into consideration the winter/summer fuel mixture and energy of a given volume of each.
 
Last edited:
:
'04 RX-8 || '10 CX-9
you're not being argumentative. :) in all honesty, i only read that paragraph, not the whole article. i was speaking only to the proposition that heated fuel atomizes (and thus combusts) more efficiently. here's the link to an abstract of a paper regarding fuel atomization and temperature.

http://adsabs.harvard.edu/abs/1987abe..symp..193W

"the overall effect of an increase in fuel temperature is to reduce the mean drop size and broaden the distribution of drop sizes in the spray. Generally, it is found that the influence of fuel temperature on mean drop size is far more pronounced for diesel oil than for gasoline. For both fuels the beneficial effect of higher fuel temperatures on atomization quality is sensibly independent of ambient air pressure."
i've been out of engineering school for a while, so i was worried that my memory had failed on that point. but smaller drop size results in a greater surface area ratio, which aids in vaporization, and thus combustion, of the fuel.

automotive gasoline engines running around sea level are hugely different from aviation gas turbines using diesel or jet fuel at altitude. the conditions both inside and outside the engines, not to mention the engineering considerations, are so different that drawing a comparison between the two isn't particularly helpful. however, i would doubt that cooling the jet fuel is done for fuel efficiency reasons; it might be a safety thing, b/c cool fuel is more stable (less vapor = less chance of unintended combustion).
 
Last edited:

9Hooker

Aviator Extraordinaire
:
AWD CX-7
but smaller drop size results in a greater surface area ratio, which aids in vaporization, and thus combustion, of the fuel.


I can drink to that!
 
:
Mazda CX-7 2007 GT
PuroMexicano said:
11.60 mpg on my third tank, I'm really pissed off, (pissed)

Well, I hope it gets better.

BTW, I have a slightly heavy foot (glare)
UDPATE

The next tank went up to 12.65.

The tank I filled out today went up again to 15.85 !!!!

Still not enough but a lot better!!!!
 

xavier

Its all about the drive!
:
Pearl White 2007 Mazda CX7 GT with Tech Package
Im still getting around 12 - 13 mpg. Ive averaged about 6 tanks so far.
 
:
CX-7
The new truth

Nay, not so!!! Sayeth the Gummamint.(bowdown)
The new stats show that most of us are actually getting what the new info shows.(sad2)

For a really good site on gas and related items go to www.automotive.com
 

CXRabbit

Proud New CX-7 Owner!
:
2009 CX-9 TR +Kenwood DNX7020EX
Actually, I'm getting more like 14 city, not sure on purely highway, but 18 combined.
 

9Hooker

Aviator Extraordinaire
:
AWD CX-7
lol @ one quick race. You get it's a 4000# brick right? And on premium gas..

The more I think about it, the more that Honda Pilot, Toyota Rav4, or even the Hyundai Santa Fe look like better options than this piece from an economic standpoint.

I still enjoy driving it just not the pain I feel at the pump for 19.1 mpg (my average over 6000 miles) premium. How in the hell do you guys get less than 16... Even SoCal stop and go/80mph stints the car has never gotten below 16mpg.
 
Top