Simulation and calculations
There is great value in being able to simulate or calculate your cars efficiency and what effects it. This allows people to concentrate on modifying a particular part of their car that is causing high losses or to evaluate potential modifications which may end up being a waste of time. The links in this wiki page show some methods of calculation and simulation.
List of tools, equations or other useful links
Ecomodder speed efficiency calculator
This online calculator will calculate your cars theoretical fuel efficiency for different speeds taking into account aerodynamic drag, rolling resistance, and braking. It will only give efficiency for steady state driving that is on flat ground.
Road Load Equation
Wiki page that explains the equations that are used in the ecomodder online tool listed above.
Gravity and inertia related equations
Wiki page that explains the effect of gravity and inertia on vehicle speed and energy. This relates to going up and down hills and the effect of car weight.
Weight effect on efficiency
Wiki page that shows calculations to evaluate energy lost when braking and how to approximately equate the lost energy to lost fuel efficiency.
A free simple online wind tunnel simulator, it can only take in a 2D representation of a car therefore it is not very detailed.
Excel calculator that characterizes braking, aero and rolling losses
This is a excel calculator posted on the ecomodder forum by RobertSmalls. It takes in a cars speed log which is logged once a second, it then calculates where the energy is lost, through braking or aerodynamic drag or rolling resistance