If I remember correctly, power steering fluid coolers were used on cars that had rear gears greater than 3.55. Is this correct? I am going to be running 3.73 gears and would like to know, do you think a cooler is really necessary? I'd rather not use one unless it's really necessary.

These coolers were used in heavy duty applications back when they were daily drivers and lubricant technology wasn't as highly advanced as it is today. IMO, unless you are using your collector car in a similar manner, I would think you could get away from using one for the lower mileage and milder use these cars tend to accumulate these days.

I tossed a factory cooler on my M-body when I originally built it with 3.55, kept it on for the 3.91 and certainly am using it today with the 4.10s.

If your plumbing has to be re-done, why not? On the other hand, if that cooler install would require major re-work, perhaps that might not be worth it.

Probably more important though is: what's the intended usage? Any long-distance huals, or just in-town driving?

The car will just be driven around locally to cruise nights and car shows. No long distance driving or racing.

Cop spec M bodies used a P/S cooler, rear gears were 2.94. So application mattress, I guess.

If it starts puking fluid put one on.

Most first installed cars I've seen with the PS coolers were cars with limited air flow under the hoods, in expected heavy city traffic. Squad cars, taxis, and the 80s 5th Ave rear wheel drive type of cars. I think once they started putting them on the cars, it solved enough issues, it became pretty standard in anything that may have been questionable.
A big block or high performance motor that creates a lot of heat, in a crowded under hood condition, that may see a lot of city traffic, would probably benefit from the PS cooler. I have one hanging on my garage wall, its been there for years.
My 49 Dodge pickup might benefit from one, there is a lot of heat under that hood, but the cooler hanging on my garage wall would have been a real pain to add under the hood. I believe the PS pump, the steering box, and the hoses are low enough under the hood that the cooler may not be needed, but I am watching the situation. .

LOL.....performance/heavy duty applications? My kids had 4 different Sunfire/Cavalier cars with 2.2 4 bangers...they all had PS coolers. They all leaked. They all got by-passed. Zero issues.

So what is the benefit of a power steering cooler? I know what it is as it is missing, rather not re-installed on my '69 GTX with 3.54 Dana 4-speed and it only takes a little driving to see.

I'd like to know what the "relationship" is between a rear end ratio and whether a power steering cooler was used.

My guess would be for highway use do to the higher RPM the pump is turning.

Still what's the negative of no power steering cooler?

Overheating of the fluid, shortening of it's lifespan, potentially loss of fluid as well. Maybe even a fire if that overheated fluid hits the exhaust manifold. Had similar happen when the trans fluid line sprung a leak and hit the manifold, poof caught fire.

That's what I thought and my GTX's power steering feels fine and responsive when I first start to drive it. After only a short time the steering starts to feel vague and very, very "light" and unresponsive and I hate it. Need to put the cooler back on.

You could probably get by without it , maybe run a syntethic fluid ? My 98,2000 and 2001 2500 diesel Rams had hydro boost brakes, along with power steering, and no external coolers of any sort ?