I have on 04 coupe 5at and was getting like 24 or so mpg now i am only getting about 21 mpg car has about 77000 miles on it. the only thing i changed was I put stock 17 on it for the winter after having 20's on it. thinking about changing plugs soon. Any other ideas that would change mpg that much thanks

 

Many places change to 10% Ethanol after 1 Nov (until spring) in gasoline. It burns cleaner, but gets less mpg. It could be nothing you've done, other than just getting pump gas...

 

It is not just the 3.3% decline from ethanol it is the reductions possible in base gasoline.

Based on gasoline in the 90's todays E10 has 11-15% lower btu per gallon, less heat less power, and less MPG.

 

Air filter might need a change if it's been on there for more than a year or 15K miles.

Winter fuel has less btus therefore worse mpgs.

Colder temps mean longer warm up which means worse mpgs.

Plugs supposedly last for 100K miles, but it's wiser to replace them sooner.

Overall, it's not uncommon to see about a 10% drop in mpg during the colder winter months, assuming you live somewhere that does see sub 60 degree winter temps.

 

I see a noticeable drop in mpg come winter with no other changes than winter tires; I suspect it's mostly the winter fuel.

 

What size 20's and what size 17's. Besides everything else that has to do with winter, your tire size changes could also cause a change in mpg, especially reported mpg.

 

I am seeing the exact same thing with my car. I have 103,000 on the car. I was wondering do you guys recommending using fuel injector cleaner?

 

its just winter gas

Gas in the winter sucks...plus with colder air you require more gas...more power too. Haven't you noticed how much faster the car feels with colder air?

I hate gas mileage in the winter since you warm the car up longer and the mileage is already lower.

 

wow didnt realize winter made that much of a difference. I live in Delaware so it getting cold here. 20's had 275 30 on rear I think and fronts 255

 

Humidity is also a factor. I never measured this with a piston engine, but with a gas turbine the power output at 100 degrees F can be as much as little as about 80% of what it is at 40 degrees F. Many think that the biggest issue is thinner air providing less lift at takeoff when you may well have considerably less thrust available. Direct water injection is actually used for a few k of hp in LM applications. Warbirds often used water injection as well on piston engines.

One thing maybe that needs to be looked at is the lower tire pressures that often get ignored at lower speeds.

 

wow you get 21mpg? i can barely do 17.

 

Mine did too. I was getting 23mpg and now getting 20mpg.... I didn't change anything

 

Yup that's exactly what I've noticed. My gas is going like no one's business and my car feels faster lol. It's a good thing I don't drive the car much.

 

To reduce costs and stretch oil supply refiners have reduced btu per gallon in gasoline [which reduces BMEP]. E10 further reduces BMEP and BTU by a minimum of 3.3% usually 5% depending on water content.

Unless you KNOW the exact btu content per gallon or pound hard to say anything about MPG.

Auto industry uses special high quality custom gasoline of known btu to conduct MPG dyno tests, so unlikely any standard pump E10 comes close. So naturally you MPG will be lower today.

 

I just close my eyes when i fill up my tank >.<

every week. 50 bucks.....