For about 10 of the past 15 weeks I've been making a one-way commute of a little over 500 miles for a work project.

Every 500-mile leg I'd been getting just about 500 miles per tank. Never pushed it all the way to 500 miles since I have no interest in running out, but it'd be very, very close.

Every 500-mile leg EXCEPT the 2 last week. Both going and returning, I would've made 500 miles easy. I was getting a rate of around 140 to 145-ish miles per quarter tank.

2009 Camry 4-Cyl, around 30,000 miles.

Same route (varied terrain), similar stops, similar speed (~65 mph avg), similar traffic (light to moderate).

No changes to tires, pressures, weight carried, etc. The first 10 trips (20 legs) were very consistent. The last two were exceptions.

Only thing different I noticed is all the previous trips were at lower ambient temp (say 20 - 40 degrees) and last week was a little warmer (say 40 - 60 degrees).

Why the sudden increase in fuel mileage? Just temperature?

 

The moon is in its closest elliptical rotation of the earth, therefore the gravitational pull is giving you a slight assist. You will only notice this when on longer drives and for some strange reason it only affects Japanese cars made in America.

 

Oh.

Well, that explains that then.

 

check your camrys thermostat. it may not have been reaching full operating temperature until the weather got hotter

 

In general, most cars tend to suffer a little in regards to fuel economy in cold weather. There are many reasons for this, but one of the most prevalent is that gasoline engines don't atomize the fuel quite as well as when compared to operating in warmer temperatures. Trace amounts of fuel are often expelled through the exhaust as temperature drops. Another consideration may be tire pressures. Keep in mind that for each ten degrees drop in temperature, the average tire will decrease by up to two pounds. During a five hundred mile commute, the drop in tire pressures will noticeably affect fuel mileage.






Terry

 

I was going to say something about fuel as well. I heard in winter gas stations use a different blend of fuel to prevent it from freezing in cold temps, and this was one of the main reasons for poor winter fuel economy. Maybe they've switched to a summer blend?

 

Maybe your local gas station is switching out of winter mixture?

 

I don't think so. 1/2 the fill ups (give or take) were in NC and half in OH. Not even sure we have "winter" blend in NC. Regardless, I doubt if OH had switched to a "summer" blend yet.


I think Terry nailed it. I hadn't changed tire pressure, but I forgot about the natural rise with temp. The atomization thing makes sense too.

 

I experienced the same thing, except i get about 300 miles per tank.

 

One of the primary reasons you get better fuel economy in the warmer months is because the air is less dense; there is less oxygen per cubic foot of it than in the colder months. The oxygen sensor reads this change as a richer mixture and the ECU leans the fuel shot to the cylinders to compensate for the reduction in oxygen density. The reverse of this happens in the colder months.

 

I must've had this backwards then.
I thought cold air resulted in higher efficiency. Is that not the premise behind modifications such as CAI's, N20, etc? Denser air, causing higher pressure in the cylinder, resulting in higher temperatures during combustion, allowing for more complete fuel burn?

 

It does to a point but then it starts to rob you of it as well past that point like Terry stated.

 

Colder air does increase horsepower for two reasons. There is more oxygen per cubic foot so the ECU must increase the amount of fuel delivered to the cylinders to keep the engine as close as possible to the ideal air/fuel ratio of 14.7:1. More fuel delivered = more power and more fuel burned. If the ECU did not compensate for this increased amount of oxygen per cubic foot of air, the mixture would be too lean.

 

I'd be less inclined to believe it's the tires for the simple fact of the friction produced while driving should make that little of an ambient temperature increase negligible in regards to weight.

 

Same here. I was getting about 250-260, however, the last two tanks I've been getting closer to 280-290. Not complaining... just noticing

 

Makes sense.
Thanks SB.

 

Another thing to add to the fuel is is where you are filling up at home does it have 10% ethanol? Is it possible where you are filling up in OH not have the 10%? I see about 60 less miles to a tank HWY when i have 10% in my fuel. It makes a big difference.

 

I get better mpg in the summer than in the winter (11L/100km vs 13L/100km).

 

Same here, been getting better MPG this past month or so than this winter. I get the best in mid-summer when it's 115°F outside. I think it's probably due to the fact I get better MPG with the windows up and the A/C on than driving around with the sunroof open and the windows cracked. All winter I averaged around 26-27mpg, but in the summer I'll usually get 28-30mpg.

 

I'm not sure the gas matters. That is, regardless of where I've been filling up, I've been getting pretty much the same mileage. Except for this last time where both an NC fill-up and and OH fill-up got me an "extra" 50 or so miles per tank.

I'm not sure about the ethanol content. The station in NC is always the same one, right by my house, just as am leaving. The return fill-up has been a bit more random - it's not always the same station.

I had also thought that cooler temps would be more effecient. But I guess what SB (and others) are saying is that more power is not necessarily more effiecient.

I was just surprised at the sudden change. I'll keep watching it over the next coupelof months and see what happens.

 

Warmer temp I just went from consistently being at 19.5-19.7mpg to 20.7. In the summer, I get 22.5-23mpg

 

I notice my car runs a hell of a lot more efficient between 50-70degrees and I get better mileage too. The first warm day we had this past week it felt great to drive, really awesome throttle response coming out of winter FTW! I notice this every year.

 

, I see a difference between the ethanol and non-ethanol also. The eastern shore in MD do not have ethanol since the EPA rules say it's not a high population area. All the Balt/DC gas stations have it and I see a 1-2MPG difference.

 

Could it also be due to less energy used by the A/C system since it doesn't have to work as hard?