In general, it depends on your plans. Some things like Hybrids or Fossil Fuel Free won't apply:
EcoModding Central
Discuss everything related to vehicle mods here, from ideas to experiments to results.
Aerodynamics
Interested in aerodynamics, or tweaking your vehicle's CdA for better fuel economy? Check in here.
Instrumentation
Whether adjusting the nut behind the wheel or evaluating your latest mod, proper fuel economy instrumentation is key.
DIY / How-to
One of the keys to EcoModding is DIY. This forum is for EcoModding DIY and How-To guides.
General Efficiency Discussion
Transportation & efficiency topics that don't really fit in any of the above.
The mods can fix any boo-boos.