Summer MPG vs. Winter MPG - Prius c version

While we have similar threads in the GenII and GenIII forums, I thought it would be a good idea to create one for Prius c owners due to the difference in this specialized little car.

The Prius c was designed to be efficient at a lower price point than the standard Prius. It was primarily designed for city/urban use. As such we can assume many people purchased the car with this type of use in mind. The following information can be used to help maximize gas mileage as well as help new owners familiarize themselves with the MPG difference between summer time and winter time driving. Please note, for most drivers who live in a climate where summer time and winter time weather conditions change drastically, there may be a large drop in MPG during winter months. If you have purchased your car during the winter season then you may be happy to learn that your MPG will likely go much higher in the summer (depending on AC use!).

First, please read this thread which details the basic information:

http://priuschat.com/forums/gen-iii...y/99492-why-mileage-gets-worse-in-winter.html

The following is a list of factors that contribute to fuel efficiency:

• Longer warmup time.

• Engine cools faster requiring more running to keep catalytic converters up to temp.

• Warm up of any kind uses richer mix burning even more fuel

• Increased oil viscosity and gear lubricant viscosity.

• Fuel Atomizes poorly in cold weather, resulting in less efficient combustion.

• Decreased battery capacity and response.

• Lower Tire Pressure (unless you check regularly)

• Increased Rolling Resistance (increased energy to flex less flexible sidewall).

• Increased Rolling Resistance (rain/slush/snow).

• Colder air is Denser meaning more wind resistance. For every 10 degree F drop in temperature, aerodynamic drag increases by 2%. In practice, MPG may increase by 1% for every 2 degree F increase.

• Winter Fuel. (Lower Energy per Volume)

• Higher Auxiliary Electric Use (Headlights, Defrosters, Defoggers).

All of these factors are important to Prius c owners but there are a few that become especially important in urban driving scenarios during winter (cold) months. Let's examine a few in more detail.

1) Increased Rolling Resistance -
Overcoming rolling resistance can account for up to 7% of total energy use in a motor vehicle. Most of this energy due to tire [ame="http://en.wikipedia.org/wiki/Hysteresis"]hysteresis[/ame] (energy lost through rubber deformation/plasticity). As temperatures decrease rolling resistance increases (takes more energy to turn roll the tire). Studies have shown that rolling resistance may be twice as high at 20C (68F) than at 40C (104F). Air pressure will also decrease in colder temps (1psi per 10degree F change) further contributing to lower MPG unless tire pressure is checked regularly and adjusted as necessary. Fuel efficiency may decrease by 1% for every 3psi under inflation!

Because rolling resistance has a larger effect at slow speeds and stop & go scenarios it only makes sense that Prius c owners should be more aware of the effects of low rolling resistance tires and properly maintained tire pressure.

As you can see from the above quote, rolling resistance plays a huge role in urban driving scenarios. To maximize MPG ensure your tires are properly inflated to at least the manufacture recommended levels (placard) or higher (no higher than max pressure listed on the tire). When it comes time to replace worn tires (4/32 for max safety or 2/32 for legal reasons) try and choose a suitable LRR tire that fits your needs. The use of non-LRR tires has often resulted in a 2-6mpg loss for GenII and GenIII owners!

2) Longer Warmup time or short trips -
Every car suffers a MPG penalty during the warm up stage. The warmup stage is required to get the engine and emissions system up to operating temperature to achieve full efficiency, reduced wear, and reduce emissions. The Prius has specialized equipment to help reduce the time spent in this warm up phase but it is not perfect. Expect to get between 35-35mpg for your first 5min (or longer) driving segment due to initial warmup. If your commute/trip is less than a handful of miles then you will be forever plagued by this warmup phase and you will be limited to sub 50mpg average unless you take steps to enable the engine to warm up faster. Short trips are a MPG killer for EVERY car and the Prius is no exception!

To reduce the impact of the warm up stage and help the engine reach operating temperature quicker you can try installing an engine block heater, parking in well insulated garage, and/or use grille blocking techniques. Do your best to refrain from using the heater/AC until the engine has reach full operating temperature. Always try and use hypermiling techniques where safe and appropriate to increase fuel efficiency.

To be continued....

 

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