the biggest problem with fuel economy is aerodynamics(as far as I've read) so i though of a way to test the car air resistance and top speed
your going to need a co-pilot for this one
accelerate until you reach you car's top speed mark it
let the engine die coast on neutral (mine also benefits from the clutch being press)
now every time car goes down 5km/h say mark, have your co-pilot write down the time (a clock with a lap time or a cel phone like a sony ericsson that has lap times could come in handy)
make a chart with it X would be velocity Y would be time
if you graph it a curve is going to emerge the steeper the curve, the more air resistance in that speed, until the curve will become a straight increasing (or decreasing line) more than a curb, that is you ideal speed band for you car, as long as you don't go over it, you'll be saving fuel
also if you measure distance and time from when the curb becomes more of a line than a curb, i think you can figure out the mass of the vehicle and with that the amount of power needed to accelerate it or keep a constant speed and that data can help you out on mods (i think :think
what do you think? could this be a good way to find out the best top speed for a specific vehicle whilst increasing fuel economy ?