Is it reasonable to get significantly higher gas mileage when the temperature is in the high 70s or 80s than when the temp is in the 40s? I drove from Madison, WI to Chicago and got 35 mpg with T in high 70s. When I drove back the next day, T in 40s, the mileage was 29 mpg. There was little or no wind (tailwind both ways). A/C on during the high mileage, off during the low. I thought with fuel injection there would little difference, but there was. BTW the car has CVT.
Temperature does affect gas mileage. So does elevation change. So does driving style and conditions. So does cargo. My guess is that temperature did affect it to some degree, but there were probably other differences between trips you did not consider.
It isn’t the air-fuel mixture (the things fuel injection controls) that is the biggest driver of fuel economy.
As @boilerengtn posts, all those other things affect mileage more. Temperature, yes, the oil is a tad thicker, the rolling resistance is a bit more but all the other things affect it, too.
One other thing to consider is that in addition to temperature affecting mileage, the winter blend gasoline also adversely affects mileage compared to the summer blend gasoline
Also the ethanol content of gasolines can vary from state to state. More ethanol = lower MPG. IL gas and WI gas may differ.
Given the rather short distance between Madison and Chicago, I’d guess the biggest factor in the variance is the action of the gas pump’s automatic shutoff, that can vary based on how fast you set it to pump the gas, and whether and how much you did or did not top up after it shut off. When the miles driven is low, any variation in the fillup process has a magnified effect on calculated MPG.
The engine management system uses a mass airflow sensor to determine air/fuel ratio.
Since warmer air has less mass than colder air, the engine will use more fuel at colder temperatures.
Tester
There’s also tire pressures. At 40, your tire pressure is lower than at 80. If the tires are inflated to specs when it’s warm, and then it gets cold, they will be underinflated which will reduce gas mileage.
True, but it only needs a given amount of air/fuel to make the needed hp, so less volume of cold air will be used (same mass of air, though). So I don’t think this is a reason mpgs drop in cold weather.
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The only effect winter has on air/fuel mixture is that the slightly longer warmup time means the mixture is a little richer, or RPMs higher, than once warmed up. That means some more fuel is being burned than if the engine were already warmed up.
The MAP or MAF system and the engine control computer supply the correct amount of fuel to match the amount of air coming through the throttle body. With denser cold air, the amount of fuel is upped, but this means a smaller throttle opening can be used; overall the density of the cold air does little to affect MPGs as far as the engine’s fuel/air management.
However, moving the car through colder denser air does require more fuel/air mix (more open throttle), especially as speeds increase. The stiffness of cold sidewalls adds a bit to tires’ rolling resistance.
Carmakers have engineering staffs who work full time on all the variables that affect fuel consumption. There must be some great technical journals on this. Anyone know an accurate layman-oriented article or two?
As far as measuring the actual difference between Madison to Chicago MPG and Chicago to Madison MPG, the ambient temperature probably has a very small impact on the actual MPG. Imprecision in measuring the amount of fuel consumed is likely the greatest factor in the different measured MPGs.
That’s a good question. I’d be surprised if a difference between 75 deg F and 45 deg F ambient temperate would cause a drop in mpg from 35 to 29. I’d expect some drop, but not that much. Your car’s computer likely does use an intake air temperature sensor (IAT) measurement as an input. But I expect once the engine reaches operating temperature whatever change to mph it might cause is pretty small. Did you do a lot of stops and starts along the way? Stopping for coffee, lunch, etc? If so, that might be the explanation, as the engine computer definitely increases the amount of gas used during the engine warm-up times, when the coolant is not up to operating temperature. And there’s be more minutes of engine warm up times when the ambient temperature is colder.
The drop seems a bit excessive to me for a temperature change.
The cars around my house (Lincoln, Sonoma, Camaro) get the same mileage in summer as they do in winter.
Don’t forget that gas expands when it gets warm so you’ll get better mileage, but that stretch of road is a nightmare with traffic and construction so I don’t think the difference is significant.
The other way. Cold gasoline has more energy per gallon, so you’ll get better mpgs with it.
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Your trips aren’t comparable, and that is likely the source of most to the difference in fuel economy. You also need to measure more than one trip under each temperature to draw valid conclusions.
I consistently get much better mileage in the summer. (Subaru forester). On the same 50 mile trip, which I do twice a week, at the same speed, I get 32-34 MPG in the summer and 28-30 in the winter.
I attribute it to the numerous differentials and CV joints all exhibiting more friction due to increased viscosity of the oil.
I don’t think gasoline formulation has much to do with this, as on a warm winter day (it hit 74ºF one day in Feb) I get the typical summer mileage.