Complicated fluid dynamics.
At a layman's level, yes, they behave more like exhaust than intake in
greater than 50% of the cases. But, the way gable vents work is that air
blowing towards one vent creates positive air pressure and the air moving
away from the other vent creates negative pressure so one vent is intake and
the other is exhaust. Maybe on a perfectly calm day they would both exhaust.
Now a ridge vent work similarly. Air blowing over the top of the vent
creates a negative pressure area on one side of the vent that sucks the air
out of the attic. The replacement air is supposed to come from the soffit
vents, thus creating the cold air flow along the rafters that is the design
effect. But with the addition of gable vents this make up air will more
likely come from the gable vents as intake, not from the more distant soffit
vents. That is why it is recommended to do one or the other but not both
when it comes to attic venting.
And none of this is true all of the time. There are many more variables like
orientation of the house, pitch of the roof, design characteristics of each
of the vents- alone and in combination, and more. Confused? Join the club.

"James Bond" wrote in message
...
While calculating the number of soffit vents I am going to install in my
house, I am operating on the priciple that I should have equal intake
square footage (from the soffit vents to be installed) as exhaust. Now my
question is do my existing gable vents count as intake or exhaust? And
just to be sure I am using the right term, by gable vent I mean the two
triangular vents at each end of my house that are right below the peak of
the roof. It is my guess that after the installation of soffit vents that
these gable vents would function as exhaust, but I want to verify that.

TIA for any help.

James