As a caretaker of two 1996 XJ RHD 4.0 rural mail delivery machines, I must interject some hard learned facts. I realize this thread is way old, and most XJ's when this post was current were in their youth. We are now 23+ years later with over 100K plus miles on the ticker .For 16+ years I worked in the last traditional auto prats jobber in Seminole County, FL.We closed in 1999. I have always been taught that on a serpentine system the distance between the two farthest pulleys, when properly tightened, should defect 1/2". My last replacement of a NEW radiator and a lightly used belt resulted in a steam storm of epic proportions as the belt driven fan fan ran into the new radiator when the bracket for the viscous fan let go. It was not pretty.Belt deflection was set at 1/2". Factory service manual wants to use a belt tension device for a used belt between 140-160 lbs tension. Been working on these XJ's since '84, never had a problem with belt tension until last month. Tensioned the belt to 1/2"deflection between the p/s and flat idler, longest run. Replaced the radiator with a new 2 row and hooked everything up. Purged the cooling system, let it come to temperature, filled the system and goosed the motor to burp the air out of the system. 'Third goose resulted in the viscous fan breaking the aluminum mounting bracket and plowing into the brand new radiator, resulting in a cloud of steaming coolant spewing everywhere, engulfing the vehicle. Not a happy camper. Gash in the new radiator was 4" long,totaled the fan, bent the upper and lower steel radiator cross braces, dumped antifreeze all over, rendering the new radiator useless. Tonight I have replaced the fan shroud, the fan clutch assembly, radiator, and all associated parts. Will advise when I get the chance as to the outcome. Peace Out!

 

I know they have to be tight so they don't squeal. My wife's 01 does not like Gates belts. If we put a Gates belt on, it squeals like Hell. Put a different one on and we are fine.

 

I run a Napa belt, which is just a rebranded Gates, on my '00.
My experience has been with my '96 XJ I used to own the "Yeah that looks about right" way is just fine.
With my '00 whole another story. Very critical about belt tension.
So much so that I bought one of these:
Kritkit II 91132


I use:
New Belt: 180-200 lbs
Used Belt: 140-160 lbs
Checking it on the longest part of the belt between 2 pulley just like terdchaser said.
And I always use the low end of the ranges.