I've just bought a new house and it has a Carrier Weathermaker 8000 2
speed gas furnace that's currently controlled by a manual
White-Rodgers thermostat (1E50W-303).

When I had the furance serviced, the tech told me that it is a single
stage thermostat and I should consider replacing it since I've got a
multistage furance.

I bought a White-Rodgers 1F81-261 multi-stage programmable thermostat
and had a question on the installation.

On the current manual thermostat, there is a green, red and white wire
connected. On the new thermostat, there is a terminal for a 2nd white
wire to activate stage 2. Do I need to just jumper W1 to W2 in order
to activate stage 2? If not, what do I need to do?

Also the manual talks about how to using the 2nd stage setting. The
manual says,

Select Stage Fast (ON or OFF) - When selected ON, the
second stage starts as soon as the setpoint temperature is
manually raised 3° F in heat or lowered 3° F in cool from
room temperature. When selected OFF, the second stage
will delay approximately 15 minutes before starting.

Can anyone recommend how I should set this, whether as ON or OFF?
Would I burn less fuel if I leave the setting off and let it run
mostly in stage 1?

Thanks.