Notice that major drop in gas mileage in the winter? There are logical reasons, a freaking ton of reasons actually.
This question recently cropped up when I took a phone call from a mechanic trying to figure out where all those elusive miles have gone, the ones that this lady felt were owed her per gallon a gas that is. We went over everything, fuel trims are normal (a measure of how closely your engine is running to stoichiometric, that perfect air/fuel ratio), tune up was fresh, tires are healthy, alignment good… air filter… mass air flow and engine coolant temp signal etcetera.
Finally I asked him “Hey fella, how is she tracking her gas mileage?” Turns out she has a piece of paper that she uses to log her mileage, she does the math every time she fills up. Impressed, I asked “When did she notice a big change?” Turns out she noticed this a month or so ago, when the weather turned from a balmy 48 degrees to -10 Fahrenheit. “Ah, this explains it. Tell her it’s the weather.” I rattle off a couple things about air density and fuel formulation that he can tell her. He calls back, she is not having it. She is getting about 19 mpg and this is a crime against humanity. Using the My MPG Estimates page I decided this was probably about right in the winter. I leave him to deal with what I think to be his real problem, a loose nut behind the wheel, if you catch my drift.
After that I took to the CARtassholes group on Facebook, where I have cultivated a group of automotive diagnostic experts, and some general rapscallions too. I asked them what they though, what drop in gas mileage is acceptable in the winter and why does it drop? These are their thoughts, they really fleshed this issue out in short order, kudos CARtassholes.
Driving Habits/Conditions: In the winter you are more likely to warm your vehicle up, this is literally throwing your fuel down the drain as it pertains to gas mileage as the vehicle is not even moving. You are also much more likely to drive slowly, to negotiate slippery/snowy terrain, and to deal with inevitable traffic jams from accidents. We all know that when one snowflake hits the ground that people forget how to drive. The average driver may not realize how much more time their vehicle spends driving at low speed, or just idling either in the garage or in traffic, this tends to accrue slowly leading up to winter and it’s a bit like boiling a frog by raising the water temp 1 degree at a time. One CARtasshole even mentioned that when the vehicle is at normal highway speeds it has a harder time staying there, the air density has increased in the cold and thus the aerodynamic drag has increased, he is our resident smarty pants. Couple all of this with the fact that the window defrosters are on, A/C is on to defog the windows, heater is on full blast… seat heaters too if you’ve got ’em, the alternator is working extra hard to keep up with the load from all these things and the poor cold battery is of less use than usual in sub zero conditions, batteries like to live in the same temperature range as we do by the way.
Fuel: Winter blended fuel is different than summer blend. You see gasoline doesn’t burn, gasoline VAPOR burns. Gasoline really likes to be vapor, you’ll notice if you stare into a bucket of gasoline (one of my favorite hobbies) that the area above the gasoline looks like it’s wavering, that’s the vapor baby! When it’s super cold outside gasoline has a hard time achieving the molecular activity to turn from a liquid into a vapor. Big oil needs to take this into consideration, and the fuel you get in the winter literally has less bang for your buck than the summer stuff, they do this by changing the literal hydro carbon chains and adding butane to help. Your engine also needs much more fuel than usual to start when it’s cold, for example when cranking on a normal summer day you might see 8 milliseconds of fuel injector operation, but in the winter this can get up to 50 milliseconds. Your engine also breathes more air than normal in the winter, air is more dense when it is cold. Most engines are not able to change how much air they mechanically ingest. A 3.5 liter engine literally inhales 3.5 liters of air per 720 degrees of revolution, if that air has increased in density there are more oxygen molecules and thus more fuel will be needed to keep the engine running at the stoichiometric air/fuel ratio.
Your poor cold car: A lot of things change on your car when it’s cold. We’ve touched on the electrical load from the accessories, how the air is more dense, how the driving habits and conditions change. But there are more literal changes to the way your car runs and operates in the winter that all add up and punch your mileage right in the face. Matter expands in all directions when it’s warm, and it’s cold it retracts in all directions. This means the air in your tires has shrunk so the relative air pressure is lower than normal unless you top it off, lower air pressure in your tires plays hell on your mileage. The bearings in your engine are also cold, the rotating components that use those bearings are harder to rotate, this is part of the reason your engine cranks so slowly when it’s super cold along with your poor beleaguered battery hating the cold. The bearings are lubricated with oil, you know this stuff, the pretty golden stuff you gotta change every couple months, this stuff is thicker when it’s cold and adds to the difficulty in rotating the engine. Transmission fluid and parts, rear differential (on some cars) fluid and parts all suffer this problem when cold. Power steering fluid, alternator, a/c compressor, water pump, everything that rotates has to work much harder than normal. Hard work = more gasoline. All of these physical changes are working against your gas mileage. Last of all there is the fact that engines are finicky little bitches, they are really only happy when they are running at normal operating temperature, this is anywhere between 195-215 degrees Fahrenheit. This is where the engine computer has the best idea on how much air is coming in, how much fuel to use, and how complete the combustion process was. When the engine is very cold the engine computer runs a fuel strategy called “Open Loop”, this is basically the computer shrugging and shitting a ton of fuel into the engine until it warms up enough for the oxygen sensors (air/fuel ratio sensors actually) in the exhaust to start reporting the results of the combustion process to the computer. Then it enters a fuel strategy called “Closed Loop”. And I know that newer vehicles enter Closed Loop faster and faster, but that doesn’t mean they are completely warm yet, and a cold engine is a very inefficient one. I drove at highway speed for 30 minutes the other day and was not yet at operating temp when I got to my destination. Keep in mind we deal with -20 degree weather sometimes.
The CARtasshole Consensus: Most of the ‘Tassholes agreed that my customer’s driver was a nut job, and that the biggest contributor to her issue is something I like to call “Automotive Hypochondria”. Some members in the group found very little change in mileage, some very great. We settled on a range of 10-30% drop in economy depending on how severe the weather is and how bad the driving conditions. I myself noticed that I drop from about 25-26 mpg to about 19-20, YIKES! 27% or so drop. Just terrible.
Last But Not Least: Never EVER underestimate the strength of cranial-rectal insertion. I had a guy who had a customer in a similar vehicle, they claimed that before the shop worked on the car they were getting 50-55 average MPG. This is of course unheard of, unless of course they only drive downhill with the wind. It turns out that they had the little digital readout on the screen set to AVG MPH, not MPG. They switched it back to MPH and the customer was happy.