Gas Mileage Mystery

Since January, I have been making the same 300 mile round trip repeatedly. Road conditions have been normal. And my gas mileage keeps going up. It has risen steadily from 22.3 to 25 with each passing trip. Any ideas why? It’s a 2003 Toyota Avalon, low mileage. I have a couple ideas but I want to see what the gang comes up with.

Maybe you have been systematically blowing out the cobwebs from previously too much stop and go driving. Maybe you are also checking your tires more frequently and adding the right amount of air.

Also, since January, temperatures have been rising. When youy start out with a cold car in January, it takes a lot of gas to get everything properly warmed up. Spring mileage is always better than winter mileage.

Please keep track of yor consumption when the A/C season starts. You will notice a drop.

The weather is getting warmer. Cold weather reduces fuel economy.

There are two reasonable explanations.

The first is that your methods of calculating mpg are crude. You haven’t told us. Maybe you do it by miles per tank, as so many folks due. For such a method, in so short a time, results vary considerably from one calculation to the next. Both of your supplied values are therefore simply normal deviations of the same mpg value and do not represent a true increase.

The second explanation is that lately you have grown more conscious of your driving habits and have recently taken to driving more conservatively. If so, it has paid off.

It’s simple!

The Mass Air Flow Sensor measures the MASS of the air entering the engine. Cold air contains more air mass than warm air. The more air mass entering the engine, the more fuel mass has to be introduced to keep the fuel/air mixture correct.


Gas sold in winter often has a higher content of low molecular weight hydrocarbons which makes cold weather starting easier but reduces both the specific gravity of the fuel and the amount of energy per gallon.

Or, it could be something else.

I had a similar experience with my old 89 Blazer - slowly over time I was getting noticibally better gas mileage - then I failed emmissions testing. My mechanic made some adjustments (maybe changed the O2 sensor - don’t recall exactly) and got the emissions back where they were supposed to be and my mileage went back to where it was before as well…

I’m sure someone can explain what happened - something to do with the fuel mixture.

It sounds like a seasonal, temperature related deal to me. If you start getting 49 MPG, I might want to buy the car. The early VWs had no gas gauge. A guy bought a new one and mentioned the great gas mileage. So. inspired, his buddies used to sneak out to his car with a gas can and top him off. The guy was amazed when he filled up after a month and took a gallon and a half. The motorcycle reserve system was great too. Whenever I saw a new bike on the roadside I would stop and tell the guy to switch to reserve and get more gas. Worked every time. Don’t say I never helped anyone.

Wow consider adding a hundred more miles to the trip. To see if millage goes even higher. You could be onto solving the higher cost of fuel- driving more? naw.

That would be a normal seasonal temperature change related mileage increase. Of course, you haven’t yet told us where you live!

I think Tester is wrong, because the mixture would produce more energy from the more fuel. Of all the answers given, I think the answers of Docnick and BLE make the most sense. Now, about blowing out the cobwebs. Yes, I do live in a moderate sized town, and don’t leave town too often. Could it take 4 300-mile trips to clean it out???
I thought maybe the tires stay colder and flatter in cold weather. Even with the same cold inflation pressure. Cold wind.

just because there is more energy per volume of fuel/air mix doesn’t mean you will get better mileage. Testers point was that more fuel is consumed per cylinder cycle leading to lower mileage in cold weather.

That doesn’t mean he is correct - but your logic for saying he is incorrect is flawed.

too many variables,IAT SENSOR ,is also a huge player,CTS. also,so on and so forth,you need to let it be. opinions vary

You know, BLE, your answer makes sense. However, Premium contains more high molecular weight hydrocarbons, doesn’t it? As in “octane”? So, my car is one of those adjustable ones, and I do get better gas mileage with premium, but only a little bit better (3%). What I am talking about is a big effect, like 10%.

If there is a “normal seasonal temperature related mileage increase”, it’s cause surely is well-understood, and we wouldn’t have all these different ideas…???

There is. Its causes are well understood.

  1. see tester’s post.
  2. your engine takes longer to warm up. Therefore it runs richer more.
  3. your drivetrain fluids are thicker, taking more oomph to move the parts through. It was REALLY noticable when I lived in North Dakota. At 20 below zero I could not even fele the gates in my tranny. The fluid was like wet cement.
  4. see B.L.E.'s post.
  5. all that snow on your roof affects aerodynamics. Just kidding guys.

Your post assumes that there is one single reason and one single driving environment. Neither is correct.

Well, it may be a combination of things. But #2 is irrelevant. This measurement is only made on a 300 mile round trip. Warmup time is irreleveant. 1,3,4 could be right.

OK. It is temperature. We made the trip today, there was a cold spell and the mileage was back down again.