What exactly causes MPG to decrease as the car gets older?

[JEDI]

2005 Mazda3 automatic

I used to 36/32 hwy/mixed when i first bought the new car.

Now 6 yrs later, i get 30/26 hwy/city. (20% decrease )

i follow all the recommended change periods in my car manual (oil, spark plugs, coolant, transmission).

i try to maintain 40psi in my tires.

So what causes mpg to decrease as the car gets older?


edit:
changed from 36/32 hwy/city to hwy/mixed

 

[Jumpem]

Right now it could be the winter blend gasoline.

 

[punjabiplaya]

shit wears out + carbon buildup inside (manifold, valves, etc...)

 

[JEDI]

Right now it could be the winter blend gasoline.

last winter i think i had 33/29 hwy/city.

so something definitely changed in my car between last winter and this.
10% decrease over 1st 5 yrs, and now another 10% decrease within a year?!

mass air flow sensor problem? i cleaned it w/maf spray a few times. no change in mpg.
how would i know if the maf sensor is failing?

 

[KIAman]

Weight...

 

[JEDI]

Weight...

even if i gained 100lbs over the past 12months, would that account for the 10% loss in such a short time?

 

[LTC8K6]

The 2.0L auto was rated 34/26 when new and is now rated 31/23 under the new rules.

The 2.3L auto was rated 29/24 when new and is now rated 26/21 under the new rules.

I would say there's little chance that you ever got near 32mpg in the city.

 

[cheesehead]

My dad's Mazda3 gets crappy fuel economy in the cold. Dunno why.

 

[KIAman]

even if i gained 100lbs over the past 12months, would that account for the 10% loss in such a short time?

I was totally joking. Being serious, most cars gain MPG as the car ages and then goes down from there. You might have simply gone over that point.

The biggest factor is reduced efficiency through increased friction. That may be internal engine wear, seals, aging components sapping power, bearings wearing out, etc.

 

[JEDI]

The 2.0L auto was rated 34/26 when new and is now rated 31/23 under the new rules.

The 2.3L auto was rated 29/24 when new and is now rated 26/21 under the new rules.

I would say there's little chance that you ever got near 32mpg in the city.

ahh... you're right.
it's 36/32 hwy/mixed
doh!

for the 36mpg, i fill up then drive from VA to NY.
for 32mpg, it's commute to/from work. mornings it's like hwy since all the lights are sync'd. in the afternoon there's traffic.

 

[Bartman39]

Things to consider would be age of the plugs and wires as they degrade over time and contribute to loss of power which also means less MPG`s... Also the condition of the fuel injectors plays a big role in how well it burns the fuel (the spray pattern is very critical)... Just these factors can very easy be the issue with a 20% fall off of MPG`s... One other very overlooked componet is the exhaust as in the converter becoming plugged up and causing excessive back pressure and even the muffler or resonator (if equipped) can fail internaly and cause excessive back pressure which for sure degrades MPG`s...

Just a note the cheap design monolithic block type converters we have these days are like a honey comb design and with excessive fuel which builds heat to the point they tend to melt in the front which plugs them up... The old bed type converters which used beads were more expensive to make, but were not near as bad about melting and plugging up but hey gotta make it cheaper eh...

 

[JEDI]

i put in new spark plugs.
hm.. didnt change the wires.

thought gas nowadays were mandated to have detergent so fuel injectors stay clean?
should i go by the red bottle of STP fuel injector cleaner?

if the converter insides isnt melted, then i can eliminate it as a reason my mpg has decreased?

 

[Bartman39]

i put in new spark plugs.
hm.. didnt change the wires.

thought gas nowadays were mandated to have detergent so fuel injectors stay clean?
should i go by the red bottle of STP fuel injector cleaner?

if the converter insides isnt melted, then i can eliminate it as a reason my mpg has decreased?

How many miles on it...? (for the wires)

For the most part just putting the off the shelf injector cleaners does not help all that much unless you do it for a period of time over and over... A product called "Seafoam" is one of the best for this method... But the best way to clean the injectors is to have a shop do it with the proper equipment and cleaner... Years ago I worked for a Nissan dealer and we would have V6 Maxima`s come in barely running and would hook in to the system with a seperate injection pump setup in a small tank (1 gal) and pinch off the return line to the main tank because the the stuff we used from Nissan was so mean it would eat the float for the fuel level sending right off its arm even with a full tank of fuel... After setting up the idle and running this stuff through the system the RPM`s would start going higher and higher in just 15 min or so till we had to lower the idle back down to normal... The engines would run smooth as silk and performance was back to par as was the MPG`s...


As for the converter you would have to remove it and do a visual inspection to be sure but some shops will drill a small hole and do a pressure test to determine if you have a back pressure problem...

 

[thescreensavers]

No do not buy stp or seafoam for gas.

Buy techeron plus, redline si1 and or gumout regane

I personally use redline and you can get it at pepboys for 8 bucks.

PEA is the ingredient that actually works and burns cleanly.

Redline contains 30-50% PEA via MSDS

I would definitely do this before paying for the machine type injector cleaner which will cost a bunch more.

 

[thescreensavers]

My dad's Mazda3 gets crappy fuel economy in the cold. Dunno why.

Cold air has more density then warm.

 

[Howard]

Wheel alignment?

 

[JEDI]

Bartman39 - 100k miles on spark plug wires.

interesting thing is that mpg actually went up to 35 hwy for a couple of tanks. then dropped to 30mpg in ~4months.
(changed spark plugs in summer)

if it's the wires, why would it cause such a drastic drop?


thescreensavers - isnt techeron the additive in Chevron gas?
and redline si1 for $5 shipped:
http://paceperformance.com/i-5154441-red60101-red-line-si-1-complete-fuel-system-cleaner-4-oz.html

 

[thescreensavers]

^
Yep the OTC version is a much higher concentration.


Wow great price, thanks for the link

 

[Bartman39]

No do not buy stp or seafoam for gas.

Buy techeron plus, redline si1 and or gumout regane

I personally use redline and you can get it at pepboys for 8 bucks.

PEA is the ingredient that actually works and burns cleanly.

Redline contains 30-50% PEA via MSDS

I would definitely do this before paying for the machine type injector cleaner which will cost a bunch more.

Would really like to hear as to why not use "Seafoam"...?
http://www.seafoamsales.com/tech-info-gas-engines/
http://www.seafoamsales.com/why-sea-foam-motor-treatment-works.html

 

[TraumaRN]

Cold air has more density then warm.

And winter blend gasoline just by itself is a 1-3 MPG decrease depending on your vehicle and driving style.

Actually with my Fusion Hybrid I can tell you exactly when they switched over to winter blend gasoline because very suddenly I lost about 2-3 MPG on the highway, and doing the math at fill up I had lost about 2.5MPG and that was before it was truly cold outside.

Now with some of these mornings when it's only 10 or 12 degrees F outside it's about a 5-7MPG decrease sometimes even worse if I have to drive into a strong headwind.

 

[thescreensavers]

Would really like to hear as to why not use "Seafoam"...?
http://www.seafoamsales.com/tech-info-gas-engines/
http://www.seafoamsales.com/why-sea-foam-motor-treatment-works.html

Oh you can use seafoam but it will be hella uneffective in the gas as it does not contain the prime ingredient "PEA" . You do not need to belive me. Feel free to read the forums over at bobistheoilguy.com

And come to your own conclusions.

I will only choose a PEA contained fuel cleaner, I wont put any other stuff in my tank

 

[Throckmorton]

The 2.0L auto was rated 34/26 when new and is now rated 31/23 under the new rules.

The 2.3L auto was rated 29/24 when new and is now rated 26/21 under the new rules.

I would say there's little chance that you ever got near 32mpg in the city.

26/21 in a compact car, seriously?

 

[zerocool84]

26/21 in a compact car, seriously?

His has a "large" engine for a compact car. A 2.3 is larger than what most compacts have.

 

[JEDI]

^
Yep the OTC version is a much higher concentration.


Wow great price, thanks for the link

i ordered one.

or do i need to do this for more than 1 tank for results on fuel injectors?

 

[ShawnD1]

-worn out bearings
-the wheels don't point as straight as they once did
-the brakes are rubbing
-inflate to sidewall?
-timing is off
-one of your spark plugs is dead
-carb or fuel injector might need to be looked at
-drag coefficient changed (adding a spoiler or roof rack will drop your mileage)
-clutch slip or torque converter slip

My dad's Mazda3 gets crappy fuel economy in the cold. Dunno why.

Winter gasoline contains more "light ends" which are smaller molecules that evaporate easier and help start the engine when it's -30. The down side is that smaller molecules contain less energy. It's like comparing diesel vs gasoline. The lighter one has lower energy density. There's also a bunch of other stuff where the engine has an ideal operating temperature and the engine will run like shit until it reaches that temperature. Old people also like to let their car idle for ridiculously long periods of time before driving anywhere (let it idle for 15 minutes before going for a 5 minute drive).