Why did my car gas mileage increase?

I have 27k on an Accord. I have not changed my driving habits and I have not done anything to the car. I used to get about 360 miles before I filled up, now I get about 400. The increase happened about 3 months ago. It’s not related to weather since my mileage in the past was the same year round.

I think this happens on other cars too, what is the reason for it?

I’m not sure why, but, if I may, I’d like to add an additional question. If someone knows why this may have happened to the OP, how can the rest of us to that to our cars?

I just bought a 2009 Jeep Patriot. I noticed an increase in my gas mileage at around 3000 miles. People have told me it is typical to see a slight increase as the engine “breaks in” whatever that may mean.

This site says

I guess my break in took 25,000 miles?

I have seen tests where a car with 20k miles will have a faster 0-60 speed than a brand new car , I guess that is also related to break in?

How are you measuring the gas mileage?

I measure it by miles driven/gallons used. I have not changed the way I measure it.

I was getting about 360/16 gallons and now it’s 400/16.

How have you determined the gas used? It could just be that the gas station you frequent has pumps that have a more sensitive shut-off trigger.

Ethanol? Ethanol decreases mileage.

Less traffic?

So it’s the trip odometer reading divided by the amount of gas it took to fill up the tank? That should be pretty accurate.

Have you replaced the tires? If so, that may be throwing off the odometer reading.

Also I’ve read that traffic congestion has reduced in the past year due to the recession and cost of gas. Is it possible you’re spending less time waiting in traffic?

I’ve been keeping a spreadsheet on the mileage for my 2005 Prius, which I bought last year. Mileage went down during the Winter, and now it is back up. I don’t know if gas is oxygenated in this state. Days are very short (and cold and wet) in the Winter, so I use the heater and lights and wipers all the time. Summer days are long and warm, so those things aren’t used as much. It’s usually cool enough that I don’t need the a/c, and am comfortable at speed with the window open an inch for ventilation. I’m guessing that in my particular car the heater, lights, and wipers draw enough power when used for the entirety of a trip to make a noticable difference in fuel economy.

Your battery is less efficient in the cold, so your gas engine has to be used more resulting in higher mileage.

Distance driven, air temperature, and type of driving makes a huge difference in gas mileage. So does manual vs automatic. I’ve been tracking the mileage on my 17-year-old Saturn SL manual car since I got it and I get better mileage now than I did when I first got it.

I average filling up every two weeks. During this past month or so, I was getting 39mpg at each fill up, except for the week I had to have my A/C on when it was really hot here and the mileage dropped to 25mpg.

I’m due for an oil change and since I didn’t have to use the A/C this past two weeks, I’m curious as to what I will be seeing in terms of mileage this coming fill-up.

I’ve managed mileage up to 41mpg several times. (I love my car!)

Im pretty sure I get better mpg in the summer than the winter. I know there are different gas formulations for the seasons, which may also be part of it. The rest probably has to do with the weather and how the car is designed to handle warmer air. Im sure the engine working harder at post-startup in the winter to warm the engine is a big gas waster too. In the summer it doesnt need to do that as much.

Article on the subject here:


Depending on where you live, climate control usage will affect mileage a lot.

In a Florida summer, you run the air nearly all the time. In a Florida winter, you don’t really need the heater much. Thus my mileage is currently down overall (though it’s perked up a bit recently because traffic is reduced while schools are out).

In say, Ohio, you might use the heating all the time in winter but not need the air con much in the summer (I’m guessing- I have no idea what Ohio summers are like).

AC use would easily account for a 10% change.

Alternatively, you may have had low tire pressures, which could cause as much as a 5% reduction in mileage.

The odometer reading would be pretty accurate unless he changed tire size or profile.

No tire change, no car changes at all. This did not happen last summer so I don’t think it’s weather related. Maybe traffic is lighter , hard to say.

It’s possible one of your cylinders isn’t firing…

I only have 4 cylinders, I think I would notice the drop in power with only 3.

I was kidding. I think you’d actually use the same amount of fuel with a misfiring cylinder since the unburned fuel would evaporate quickly and be ported through the exhaust.

Hopefully GaryT or Rick will be along shortly to correct or confirm.

A 14 mpg drop in economy because of A/C? That seems rather high. I only get a difference of about 2 miles per gallon that I’ve noticed. Does it differ that widely between vehicles?

If I’m not mistaken, I don’t believe it really matters how much you use the heater, wherever you are.

The heater in automobiles uses waste heat from the engine. The heater core is just another radiator, so far as the engine is concerned.

The only energy draw would be the use of the fan.

So far as the OP is concerned, my money is on different fuel formulations (winter vs. summer). Another loss of efficiency in winter (besides those already mentioned) might be the increased viscosity of oil/grease in the various moving parts, at least before the car warmed up.

Yeah, but if the heater is off the engine doesn’t have to circulate coolant through it. I thought that accounted for the majority of the heater’s (or air conditioning’s) power usage.