Just as a data point my 92 Dodge Caravan was climbing the North face of
the Grapevine here in So Cal on a 95 degree F day with the AC on. The car computer 
said 9 MPG with the cruise control set at 70 MPH or so. That is 7.77 gals per 
hour or 46.66 pounds of fuel per hour. Divided by a BSFC of .45 gives right 
around 100 engine HP at 3800 RPM.  Half a dozen or a dozen vehicles were parked, 
overheated, on the side of the freeway going up the mountain.

The Dodge rad is 15 by 17 by 1.5 inches thick or 255 square inches. 
Rad volume is then 382 cubic inches. The  15 inch diameter cooling fan 
draws 15 to 20 amps at 13.5 volts or 230 watts or about .3 HP. Figure 1/4 HP
taking into consideration about 80% DC fan motor efficiency.  

The engine started to overheat, or the thermostat could no longer
maintain 195 F, until I slowed down to 60 MPH, 13 MPG and turned off 
the AC.

Paul Lamar
 
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