One question that comes up in any question of fans is:
'What is the cfm of my stock fan or this electric replacement'.

Most of the aftermarket electric fan companies give a cfm that is in 'free air' without pulling through a radiator, which is a kinda worthless number.

Last night i happened to be in Target
and noticed for the first time
that Target was now selling a $30 Eddie Bauer brand
"Weather Watcher" which was a combination
portable wind speed gauge and air temperature reader:

http://tinyurl.com/22nxvf

This device could be used to measure your cfm through your radiator
by moving it all around and across the radiator to find an average
wind speed in feet per minute,
then mulitplying by the height of the radiator in feet,
then multiplying again by the width of the radiator in feet.

It is even possible that the 'guts' of this electronic device could be mounted in the cab of the truck and by using extension wires the tiny windmill mounted in front of the radiator so that cfm could be measured while driving.

Kestrel makes similar wind speed devices but they are several times more expensive.