CarTalk.com Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Why better gas mileage in 100 degree weather than in 35 degree weather?er

1996 Mazda Protege, 150,000 miles. Gets 27mpg in heat of summer and barely 20 in cold of winter.

Why?

Better gas mileage in the summer is related to gasoline oxygenation for the most part. But the difference is usually about 10%… You have about a 25% decrease, and that seems high. What maintenance does the car get? How often do you change filters and oil? I assume that you are driving on the same roads with the same amount of traffic.

…or maybe the OP is one of those people who warms the engine for several minutes each morning in the winter…

Thick lubricants, stiff tires, warm-ups, headlights, heaters, wipers are on more of the time…It all takes fuel…

Another reason is that hotter air has less oxygen in a given area - so each time your cylinder sucks in air, it’s getting less oxygen than it does in the winter. The computer sees this and reduces fuel delivery accordingly.

Also the cars motor will take longer to warm up in the cold weather. Thus keeping the car in open loop for longer (ignoring the o2 sensors) and running the motor rich.

If there’s a lot of stop and go driving in the winter because of icy winter and snow plowed roads, that might be a potential cause. I noticed this too when I lived in Colorado at 6500 ft. I think is was partly due to more warm-up time, and longer need for choking, but mostly because the driving speed was closer to 30 mph in the winter, and more like 50 mph in the summer, simply due to the driving conditions. You couldn’t go very fast on most winter days even if the roads have been plowed & sanded b/c often the snow plow kicks rocks into the roadway, so you have to go slow enough to avoid these rocks in the road in the winter. You learn this fast. One oil pan hole, and that makes for a bad day you don’t want to repeat. In the summer, no problem.

How old is the thermostat?
If over 6 years it’s time to change it.
That could close the gap somewhat.
I recommend getting a replacement from Mazda, not aftermarket.

  1. I do regular maintenence, oil change yearly and check ups at a reliable independent mechanic. I check the fluids monthly and top up if needed.

  2. I do NOT warm the engine on a cold morning. My driving is not rabbit starts, etc. I drive mainly in town and an occassional I-road trip at 75 mph.

  3. I am a second owner to the car and I have never had the thermostat replaced and the previous owners did not as well.  That will go on my list of questions for the next time I take the car in.
    

Thanks to all who responded! All of the explanations seem reasonable.

what weight of oil our you usen in the winter that will make aq big differnce and watch the tire pressure as it gets colder your tires will loss pressure.

Between what Caddyman said…and the winter gas formula…there could be a significant difference. Not all cars are created equal. Some won’t see much of a difference. Some will see drastic differences.