F8L's advice is very sensible... 1) ScangaugeII => learn to hypermile, this can increase your overall fuel efficiency up to 10 mpg for most drivers and lower your carbon foot print all year round. cost $160. 2) LLR tires = adds 3 mpg to your drive. retail cost estimate $500 However, if you are willing to go further than that consider the following... 1) 12vdc tire air pump & high quality dial or digital tire pressure gauge -> raise your tire pressure to 40psi front, 38 psi rear. This can increase your overall fuel efficiency all year round. cost $50 2) 3" diameter foam pipe insulation=> create front air take insulators/grill blocking pipes for low speed driving when the temperature drops below 55 degrees Fahrenheit. This helps increase fuel efficiency during the winter. cost $3 3) Tint your windows or get PriusChat customized Heatshield Sunshield or a driver's cooling vest => to lessen any need to turn on the AC when you are driving in the summer heat. This can help increase fuel efficiency in the summer. cost is variable. 4) 400W Engine Block Heater (+ electric AC timer, AC extension cord) => can increase fuel efficiency during the winter time. Lifespan estimate 4 years. cost $60+installation cost + higher electric bill 5) Enginer Plug-in conversion. Lifespan estimate 6 years. cost over $8000+ higher electric bills 6) shark fin external antenna replacement => small FE ROI by lowering aerodynamic drag(*) at speeds >55mph. cost 78 7) Aluminum Alloy wheel rims => small weight saving EF ROI but looks cool. cost variable. (*) All of the currently available 2010 Prius Aero(dynamic body modification) kits are tuned for performance/racing and not for fuel efficiency.