Winter Fuel Economy Reduction – Causes and Helpful Hints

Mike McGinnis

Each winter we at Innovative Tuning see posts on the internet and field concerns from our customers who often think they may have a significant issue with their vehicle based on a change in fuel economy. In the vast majority of cases there is nothing wrong with the vehicle. Instead there are a number of factors combining to reduce fuel economy. Rather than just saying it’s cold out and that’s why fuel mileage is poor, I’m going to explain how colder temperatures result in reduced efficiency in hopes that it will put people at ease and help them understand that much of the reduction in efficiency is unavoidable. Last I’ll provide tips to help you prevent some of these losses and hopefully save you some money. If you live in a climate that doesn’t get cold and experience snow and ice this article is not for you.

First I’ll cover things that are inevitable:

  1. Increased aerodynamic drag – Car companies invest millions to tune aerodynamics for a combination of stability at speed and low drag. Despite their best efforts they can’t avoid the fact that air density increases as it gets colder. More dense air causes more drag. We can’t see this so I’ll give you an example that will hopefully help you visualize this. Think of conventional engine oil which is thick when cold,  but pours quickly when hot (more on this later). Then picture wading through a pool of cold oil vs. hot oil. Don’t picture the skin cancer or burns this may cause,
    just how much more energy it would take to walk through the cold thick oil vs. the hot thin oil. In vehicle terms as the car cuts through the more dense air you require greater engine output to achieve the same speed and higher engine output requires higher fuel and air consumption. Air density increases ~20% when the temperature goes from 90 F to 0 F (other things being equal). Aero drag is directly proportional to the density of the fluid or gas an object is passing through. This means a ~20% increase in air density results in a ~20% increase in aero drag. This increase in aero drag reduces fuel economy more at highway speeds where aero drag is more significant than during around town lower speed driving.
  2. Increased idle time – When your car is idling you’re using fuel without going anywhere so you’re getting 0 MPG. In the winter as we let our engines warm up before loading them during driving to reduce engine wear. This increase in time at idle increases fuel used before we start driving and drags down your average MPG.
  3. Lower average engine temperature – When it’s cold out it takes longer for an engine to warm up. Engines are built with clearances that are meant to be optimal when the engine is warm. When the engine is cold clearances are larger because components have not yet expanded to their normal operating clearances. This extra clearance results in greater emissions and reduced efficiency. MPG is further decreased due to lower average engine temperature because engines are tuned to operate at richer air/fuel ratios during warmup. This aids in idle stability and smooths engine operation if an operator begins driving while the engine is still cold.
  4. Engine and drivetrain lubricants are thicker when cold – As mentioned above if you’ve ever drained hot oil from a car vs. poured it cold you know it’s thicker and resists motion more when it’s cold. If you haven’t done that perhaps you’ve put cooking oil in a skillet and moved the skillet around to spread it out then noticed how the oil moves around much more quickly/freely when hot. Your engine, transmission, differentials (and other drivetrain components i.e. transfer case if applicable) are oil lubricated. Until these oils come up to operating temperature they’re thick and create significantly more drag on moving parts. Again this increased drag means more engine output is required to move the car forward and increased engine output means increased fuel consumption. In addition to the oil lubricated components your wheel bearings and other grease lubricated rotating parts move less freely while the grease is cold and more dense. Think of heating up hot fudge so it runs like water and then watching it get thick when it gets cooled by your ice cream.
  5. Increased rolling resistance due to temperature change – In addition to the engine and drivetrain experiencing greater drag from thick fluids your tire to road interface is experiencing increased rolling resistance in the cold. This change does not have a massive effect on fuel economy but it’s worth mentioning.
  6. Increased electrical loads – I’ll keep this section simple as well since this doesn’t have a huge affect on fuel consumption. Cold weather starting drains the battery more so the alternator needs to charge it more. In the winter we run the electric defroster(s) we use our headlights more because there are fewer hours of daylight and we use the heater a lot. If you use the a/c all the time in the summer that balances with the heater usage. If you have heated seats mirrors front windshield headlights etc. those things all add to the electrical load. As electrical demands increase the alternator needs to charge the battery more and the alternator puts more drag on the engine while it does this. Again increased drag on the engine means more engine output is required so more fuel is consumed.
  7. Winter gas – In the winter we are provided with altered gasoline formulas. They use a high vapor pressure formula which helps startup but is less optimal in terms of drivability and efficiency when the fuel is warm. These fuels increase engine deposits and fuel dilution. Engine oil should be changed more frequently in the winter. If you’d like more info on this Chevron offers information to the public on their various gasoline formulas and other suppliers may do the same.
  8. Road conditions – Driving through slush snow and ice increases rolling resistance so the engine has to provide greater output for the tires to push through it. These conditions often results increased traffic which results in reduced fuel economy as drivers speed up and slow down over and over. It’s even worse in stop and go driving.

Here are some things you can do to help your fuel economy:\n

  1. Tire pressures – If your vehicle has a recommended tire pressure of 35 psi and you drive with tires at 28 psi you increase rolling resistance about $12% according to a study by TireRack. Tire pressure changes about 1 psi per 10 degrees Fahrenheit so it’s easy for tires to be come under inflated as it gets cold. Some air leaks out of your tires over time as well so keep an eye on tire pressure because this is an easy way you can avoid a significant reduction in MPG.
  2. Engine block heater / Heated garage – Engine block heaters warm the engine via an electrical heating element which you plug into a wall outlet. As it heats the engine block it also warms the coolant and oil. It reduces the density of oil and coolant which reduces drag on the engine. It improves the efficiency of combustion because the cylinders are warm to start. It reduces engine warmup time which reduces time spent idling at 0 MPG. \nOn a side note this also reduces engine wear which occurs on cold starts. A heated garage won’t get the engine block and coolant/oil as warm as a block heater but it affects the whole vehicle so all fluids bearing grease tires etc. start at a higher than ambient base temperature which will improve fuel economy too.
  3. Narrower but taller snow tires to maintain tire diameter – Snow tires that are a bit narrower than your stock size will work better in snow and slush by increasing downward pressure and reducing the plowing effect as a wider tire pushes through snow. This improves traction and reduces contact area which reduces rolling resistance with the road and plowing resistance as the narrower tire goes through snow. Lower resistance means better fuel economy due to decreased engine output being required.
  4. Wash car / Spray it off – Snow and ice collects on vehicles which increases vehicle weight requiring greater engine output to move the vehicle. In some cases snow and ice can build up so it rubs the tires which increases rolling resistance. Snow collecting on the under body and top side of the vehicle also disrupts the aerodynamics of the vehicle which increases aero drag. Spraying the snow and ice off reduces extra weight and returns the aerodynamic profile of the car to normal.
  5. Engine braking – On vehicles equipped with manual transmissions engine braking while engine RPM is above a certain point shuts the fuel injectors halting fuel consumption. Engine braking may also provide increased vehicle control when compared to braking while in neutral. This works all year round.We feel idle/warmup time lower average engine temperature winter gas and reduced tire pressure are the leading causes of reduced fuel economy out of all the causes listed. We feel maintaining proper tire pressure and using a block heater are the two ways you can best reduce winter MPG losses. Beyond that it’s hard to prioritize the lesser factors because they’re vechicle/situation specific.

Have fun, be safe, and happy holidays from all of us at Innovative Tuning!

written by: Mike McGinnis