I believe it is the same as the MODE button on later transmissions. When engaged, the transmission will go to higher gears quicker so the engine doesn't rev up as high. It also holds a higher gear longer before downhifting. Lower engine speed equals less fuel burned but it also may mean lower power. If you're in the country, I'd turn it on and monitor the fuel mileage for improvement. Every time you shut the key off you may have to turn the Econ switch back on.