For those of you who drive stick, upshifting at lower revs will use less gas; consequently, driving in a higher gear will use less than driving in a lower gear, as your revs will be lower.
Around town, I usually upshift at just above 2000 RPM; sure the engine produces less torque at lower revs, but since you're not racing around elementary schools there's no need to gun it at low speeds.
I use the rule of thumb to keep my RPMs always between 1500-2500 RPM, as I feel that's the best compromise between torque and fuel economy. Thus I'll be in fourth gear even just coasting through my neighborhood.
Note though - when you are traveling up hills, high gears will actually reduce fuel economy, as your engine has to struggle to climb the gradient with low torque. So everything I said above should apply to level grades only.
by realgeneric on February 13 2010
this tip works for 90% of voting Fuelly members.