What can cause the fuel efficiency to decrease alot?

[NewGuy123]

My fuel mileage has gone from around 26mpg to 20mpg recently (8-9l/100km to 12-13l/100(canada))

2010 Mazda 3 2.0L automatic. I am doing the same driving everyday (commuting) and driving habits have not changed. I went to get the oil changed at the dealership a couple of weeks ago and I have noticed the difference since then. No warning lights and the vehicle feels around the same. Any ideas on whats going on? (pissed) Thanks

 

[BRIAN_MP5T]

Dirty Air Filter
Dirty Spark Plugs
Bad Gas
Driving with the park brake on
Low Air pressure in the tires

It's becoming winter.... thus it's becoming cooler... thus your fuel economy will change....

 

[njaremka]

if it happened right after the oil change, double check that you HAVE oil in the engine and how much is in there. its also possible they put the wrong weight oil into the engine, but that will be harder to verify. these are the reasons i perform all my own oil changes.

 

[bikingpro5]

winter gas already being shipped to the gas stations, i always notice worst gas mileage once it turns colder

 

[NewGuy123]

Dirty Air Filter
Dirty Spark Plugs
Bad Gas
Driving with the park brake on
Low Air pressure in the tires

It's becoming winter.... thus it's becoming cooler... thus your fuel economy will change....

Dunno about the first three but definitely not the last two. I guess I can at least check out the air filter pretty easily.

if it happened right after the oil change, double check that you HAVE oil in the engine and how much is in there. its also possible they put the wrong weight oil into the engine, but that will be harder to verify. these are the reasons i perform all my own oil changes.

Well I drove about 800km on the highway after that oil change, so if there was no oil, wouldnt the car have seized by now?

winter gas already being shipped to the gas stations, i always notice worst gas mileage once it turns colder

Maybe its this. I am getting around the same mileage as I was getting last winter. Then again that was when there was ice on the roads, I had winter tires on and it was -20C with the heater blasting non-stop. Right now, the driving conditions aren't so different from what they were two weeks ago. Would a different gas blend make that much of a different more than 25%?

 

[puredrive07]

I noticed my fuel economy went down the toliet last month (August), it was super hot the whole month and I contribute the bad MPGs because of the heavy A/C use. I have seen it crawl back up to the high 20s to low 30s again. Driving habits only changed slightly for me, I drive a little more city now, but definitely not urban city that is stop-n-go traffic. My Volvo had the same problem last month and it bounced back up +1-3mpgs better this month.

 

[ToNeZ90]

i usually notice a very slight increase in my mileage. during the winter. idk why but it must be because of my driving habits.

On a more on-topic note, i'm in FLA and its starting to get "cool" out. I did a uhh.....HIGHLY spirited 50min drive around the town and after parking my tires were barely luke-warm. SOOO that means that my tire pressure has prob gone down a bit. I havent done my normal pre-winter check yet so i prob lost a mile or 2.