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Amazing temperature dependance of gas mileage

My 2003 Toyota Avalon gets vastly different highway gas mileage depending on the temperature. Literally: at subzero temperatures, it gets about 22. On a warm day, it can get 29. Same road, same speed, same driver…

I know its not gas formula, because I have seen the value go up morning versus afternoon on the same day.


If you’re using the on board trip computer, that’s not the most accurate way of measuring fuel mileage. To do it right you have to average the results of 3 or more entire tankfuls of fuel, it’s unlikely you’re going through that amount of fuel in an afternoon.

Cold weather results in a lot more friction to ovecome. As well, if the road suface has snow or slush on it, that results in much more rolling friction. Your driving pattern may change as well with many more short trips in cold weather. If most of your driving is short trips, the dramatic difference could be true.

However, neither should result in such a large mileage drop, if, indeed, you measured this ACCRATELY over several tanks of gas from the SAME PUMP.

I would measure some more as indicated, and if you are SURE there is such a drop, take the car into a good shop that can diagnose faulty sesors.

A few questions:

  1. How long of a drive is this? 1 mile, 10 miles, 100 miles? If it is a short drive, I’d expect the difference to be exaggerated, as the engine may get to normal operating temp in warm weather but not in cold, and the fluids will be that much thicker. Not necessarily this big of a difference, but certainly more than you would see over a 100 mile drive

  2. How are you calculating fuel economy? If by the computer on the dashboard, forget it for a short hop - they aren’t nearly as accurate as calculating by hand over several fillups

If you’re really experiencing that big of a difference, I’d check the intake air temperature sensor and after that, maybe the coolant temperature sensor to see how they’re doing.

It’s going to fluctuate some with temperature, and accounting for error on the computer you’re probably only fluctuating 3-5mpg, which isn’t really all that surprising. As the temperature gets colder, the air gets denser, which means more of it gets into the combustion chamber. And that means the ECU will dump more fuel in there to maintain the proper mix. Hence, worse mileage (but more power).

Is the engine reaching normal operating temperature quickly?

If the theremostat is stuck open, slowing engine warm-up, poor gas mileage will result.

Lower air temperatures also mean reduced tire pressures, and lower pressures reduce fuel mileage.

Under normal circumstances you cannot accurately measure a difference in fuel mileage from morning to afternoon. Where are these mileage figures coming from?

As I said, these are long road trips. It is from the onboard computer, but
averaged over many trips with the same result. The distances involved are 100 miles or more.

There is no check engine light. This is measured on dry roads.
I find it hard to believe that tire pressure and fluid (synthetic) viscosity could vary enough to cause a 25% drop in gas mileage.

It is still likely that the on-board records are not accurate. The only way to check that out is to manually measure several fill ups. See how close they are. On board computers are good guesses, but how good depends on a number of factors. Sometimes they are far off.

Subzero temperatures can absolutely cause that high of a drop in gas mileage.

In addition to lower tire pressures (roughly 1 psi for every 5 degrees), all the drivetrain lubricants are like molasses, and the engine will take substantially longer to reach operating temperature…if it does at all. And cold engines run richer. When I lived in North Dakota, we used to have to put cardboard in front of the radiators to get the engines to come up to operating temp on the highway at subzero temperatures. At highway speeds that onrushing subzero air is drawing the temperature out of the coolant almost as soon as it starts going through the radiator tubes.

All the fluids are synthetic–aren’t they much less viscous when cold?
Also, the coolant temperature gauge reads absolutely the same.
I take your point on tire pressure. The damn thing has the new silly tires
(60 series) so a small loss of air causes a lot of temp drop.
I do keep them pumped up–I do add a lot of air every fall.

Synthetic oils are still pretty thick when its really really cold out.
The main issue is still the radiator bleeding off the heat the engine needs, and the thermostat staying mostly closed because of the coolant cooling off so quickly.

When the thermostat is allowed to stay open all the time, the water pump doesn’t have to work nearly as hard, because of the resistance in the coolant flow that the thermostat creates when it is closed.

That drag on the coolant pump requires the engine to work harder to keep spinning the serpentine belt on the front of the motor, which contributes to the loss of fuel economy that you are seeing.

So not only is there a loss with the extra friction created by the colder air that you have to push your car through at highway speeds; and the extra friction created by the cold, thick fluids that your engine and transmission have to spin through; but the water pump has to work harder because the thermostat is staying closed most of the time.

All of these factors add up to less fuel economy.


Those sound like reasonable arguments. I should clarify that I am seeing this at tempertures like 30 degrees Farenheit. Kansas, not North Dakota. Range of 22 to 29 winter and summer.

Doesn’t really matter.
I live in the Denver area, and see the same temperature swings that you do in Kansas, and I see the same fuel mileage drops in my different vehicles when the cold weather arrives.

At least you only have 1 vehicle you are keeping an eye on.
I have 6 cars, and 4 motorcycles that I keep a watchful eye on for changes in behavior.


Cold, dense air gives more wind resistance.
A 46 deg. F drop increases density 10%.

An old sticky thermostat might let the engine run cooler in cold air.

The colder, dense air going into the engine requires the throttle to be more closed, for the same amount of power.
That increases pumping loses.

This then leads me to wonder at what temperatures the EPA gas mileage spec is supposed to be true for?

“viscous” means “resists flow”. Synthetics are more resistant to flow, more viscous, when cold, just as regular oils are. Synthetics are chemically the same as dinos. They’re considered better for extreme temperatures primarily because the molecules are more consistant in size, giving them better lubricating properties throughout the temp ranges, and at extremes better is considered…well, better. Synthetics also have fewer impurities, and at extremes that’s also considered an advantage.

The 60 series tires, are they a change in tire size from the originals? If so, exactly what are the old sizes and what are the bew sizess? You may have introduced a reading error.

My comment on this issue is as follows:

My typical gas mileage is so much of a constant that I sometimes think the digital readout of my average gas mileage is stuck on the same reading. However, manually calculating my gas mileage confirms that it is extremely consistent. Over the past two months, including some REALLY low temperatures, my average mpg reading has been 21.3-21.7, which represents very little variation. It should be noted that my typical driving pattern/speed differs very little, so the major variable is the ambient temperature.

That being said, in the wonderfully-higher temperatures that we have had in NJ over the past couple of days, my gas mileage has suddenly gone up to 25+ mpg, on average.

Yes, I know that the only way to accurately assess gas mileage is to average it over the course of at least 3 tank-fuls, but the sudden rise in my mpg is consistent with the sudden rise in ambient temperatures. I will continue to monitor the gas mileage figures for my car since I have only been driving it since 9/1/10, but at this point, it does appear that the ambient temperature does play a significant role in the mpg that this car can achieve.

Incidentally, for an AWD vehicle with 3.6 liter/256 HP engine, used in local/suburban driving (and including a few…vigorous…acceleration runs), I think that 25 mpg+ overall is not too shabby.


Up here in NH mine has always dropped in winter. It simply takes more miles for the engine to get to operating temp. When I lived in North Dakota the struggle wasn’t with gas mileage, it was with trying to get the engine to stay at operating temp on the highway. Subzero temperatures at 70 mph speeds could pull the heat from the coolant faster than the engine could replace it. The T-stat tried hopelessly to regulate it in some stable manner, but to no avail.

Nope, but you’re close.
It is a 2011 Outback Limited.
The Tribeca engine is optional on the Outback, and with the reduced weight of the slightly smaller Outback it accelerates effortlessly and gets decent mileage.