# Drag Coefficient and Snow, dirt, or hail damage

## Main Question or Discussion Point

I have a general question concerning the affect of surface treatments or imperfections in the painted surface of the vehicle in relationship to the Drag Coefficient for an automobile.

I own a 2006 Hyundai Sonata which has a drag coefficient of 0.32. Over the last six months I have noticed a slight reduction in gas mileage in the range of 10-20% on our normal commute which averages out to be 4.5 MPG drop. Several things have happened in this time frame including winter gasoline formulations and severe damage from a hail storm.

I understand that the colder temperatures in the winter can reduce mileage based on several factors, but I am interested in the drag coefficient affects in the following scenarios:

1. ambient temperature of 5 degrees and 30% humidity vs. the standard temperature and humidity used in vehicle testing.

2. possible affect of hail damage. Dents are an average one half inch diameter and one eight of an inch in depth. I have included a couple of pictures that show the damage.

3. affect of snow buildup on a car both from falling snow that has sat on the car all day is not removed prior to driving and snow that is picked up on the bottom of the car and behind the wheels.

4. the minor affect of driving a dirty car vs. driving a clean car. I am not sure what the friction due to a dirt covered car would be, but I am sure it would be higher and would affect a reasonably aerodynamic car more that a truck.

These situations would have the most affect at highway speeds which constitute a majority of our drive, but this discussion is in regards to the coefficient. If this can be figured out, the speed of the vehicle can be worked into the drag equation.

Thank you

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1. ambient temperature of 5 degrees and 30% humidity vs. the standard temperature and humidity used in vehicle testing.
Well.. as the air becomes colder, it gets dense and it might reduce the fuel milage.. but the effect is too low, especially for the average velocity of 30-40 kmph of a car to result in a 4.5 MPG drop.

2. possible affect of hail damage. Dents are an average one half inch diameter and one eight of an inch in depth. I have included a couple of pictures that show the damage.
Those dents again.. are too small and scattered. Their aggregate somehow seems to make up for an aerodynamic shape. The result will be even smaller than for the first case.

4. the minor affect of driving a dirty car vs. driving a clean car. I am not sure what the friction due to a dirt covered car would be, but I am sure it would be higher and would affect a reasonably aerodynamic car more that a truck.
For that to have any effect at all.. u need to drive with a thick layer of unevenly spread layer of mud and have metal thorns protruding from your wheels.. which doesn't seem to be the case.

However, dust particles, dirt when stuck between moving parts like gear systems, axle systems or such high spinning devices and other parts within the car, they can pretty significantly affect the performance. However, it is difficult to produce this effect in form of a mathematical equation.

These situations would have the most affect at highway speeds which constitute a majority of our drive, but this discussion is in regards to the coefficient. If this can be figured out, the speed of the vehicle can be worked into the drag equation.
In the real world.. the interaction of your car and the fluid [the air] results in a pretty dominant phenomena. As such, the changes you mentioned above will have absolutely no measurable effect.. because any change could just as easily be attributed to change in testing conditions. Because those testing conditions will produce a change much larger in magnitude than what the abovementioned conditions can.

These were my general thoughts as well, but this one is a little hard to figure out. I have little experience with drag except for how it relates to fluid dynamics in open channel flow for pipes and drainage channels. Thank you for a quick reply.

Average speed

I am not sure exactly what is going on with things up here in South Dakota and my Hyundai Sonata to be specific, but I plan to inquire with some automotive forums that can better help with the drive train on this vehicle. wehn the vehicle turned 60,000 miles, the mileage went right back up to where it was. I still think there may be some influences due to temperature and our average speed of the vehicle.

Our average vehicle speed is 47mph or about 75kph. 60 miles of our daily commute are at 70-80 mph or 110-125kph. At these speeds, I am curious if the increased resistance would be significant enough to start making a difference. I wish I could find my Fluid Dynamics text book so I could figure it out myself, but it is MIA in my crawl space.