Ray Chez: You make some interesting comparisons but missed on the HP used. A rear rad usually runs constantly. No variable speed. Thus it is using HP all he time. A side rad only draws HP when the fan is running. And the fan speed is variable.
Not entirely true. This would be the case for older coaches. But starting around 2008 Cummins started using electric fan clutches on all their rear radiator models. ISB, ISC, & ISL. I have also seen some late Cat C7's with fan clutches.