Tom,

Measure the return air temperature near the furnace with an accurate
thermometer. Your thermometer should read degrees and tenths of
degrees. Take at least 2 readings and get the average. Write it down.

Measure the supply air temperature near the furnace the same way, Take
at least 4 readings and get the average. Make sure all readings are
out of direct sight of the heat exchanger. Write that temperature
down.

Subtract the return air temperature from the supply air temperature.
That is the Temperature Rise or Temperature Difference (TD). Write it
down. Get theBTU per hour OUTPUT rating from the furnace data plate.
Write it down. Put these numbers in the following formula:

BTUs Output / (1.08 * TD) = CFM

This is the ACTUAL CFM of air flow. Note that each fan speed has a
range of air flows. This is called a fan curve. It is usually shown
as a graph or a table. For every pressure difference across the
furnace, there will be a different air flow even though the blower
speed connection has not changet. Each speed connection has a different
fan curve. If you use all the fan speed connections and measure the
air flows as shown above and plot them over the fan curves, you will
get the system curve. The air flow will always be where the system
curve crosses the current fan curve. A duct system with a higher
resistance will result in a lower air flow. When you increase the
resistance, the blower amps will go down! Try measuring the blower
amps and partly covering the return with cardboard to demonstrate that.

If you cannot make the measurements and tests I outlined above, DO NOT
try changing the blower speed connections. You should be able to
measure the results of your efforts if you are going to make any
changes.

Hope this helps.

Stretch