brief guide

[fabrio]

To Change or Not to Change

When you modify your car in any way, you are changing the original design and the vehicle's overall characteristics. Sometimes that is a good thing, while other times it can lead to negative results. Below is a list of common modifications and their estimated change in percentage of the overall drag coefficient (Cd) of an average car. For example, the outside rearview mirrors add a two to four percent increase in the Cd. However, it should be noted that your results may vary.

Modification % Change

Lowering the vehicleby 30mm approx. -5
Smooth wheel covers -1 to -3
Wide tires +2 to +4
Windows flush with exterior approx. -1
Sealing body gaps -2 to -5
Underbody panels -1 to -7
Concealed headlamps +3 to +10
Outside rearview mirrors +2 to +5
Airflow into the enginecompartment +4 to +14
Brake cooling devices +2 to +5
Interior ventilation approx. +1
Open windows approx. +5
Open sunroof approx. +2
Roof-mounted surfboard rack approx. +40

from : http://www.europeancarweb.com/tech/0...de/tables.html