Honestly, I think you should get multiple opinions on this. The thing is, it really comes back to what YOU feel comfortable with. (Not to change the subject but recently I posted about buying a 1995 F450 and some thought I was basically out of my mind and it wasn't worth even close to what they were asking and others understood that I wanted for hauling and didn't care about looks and thought it was a pretty good deal).
So, people have different thoughts on what is extreme or what is acceptable or normal.
Here's my take though:
If you're hauling a car trailer with an average car on it and the total weight is about 6,000 to 7,000 pounds (2,000 or so for trailer and 4,000 or so for car), then I would say you should be ok with this without an additional cooler IF...IF you're driving on "mostly" flat terrain with only a few large hills here and there.
But, if it's in mostly hilly or mountainous terrain like say between Denver and Las Vegas (mostly ALL mountains except for the last 50-100 miles to Vegas) then I say you need an additional cooler even hauling a small trailer or car trailer. Heck, just the pickup alone without a trailer and I would consider it. Especially if I was traveling a lot of distance in that area every day.
The reason I say this even for just a pickup is because going up and down a mountain area will cause constant downshifting and upshifting and back and forth and back and forth. Over time, it will create more friction and heat which is what we all know we want to prevent.
So, I would "consider" it even for just a pickup.
Now, if you're hauling something heavier like a large pickup on a trailer or more weight than a car trailer (8-10k plus) then I say you should probably put a cooler on). Anything above this and I say it's almost a must.
But alot of this will come back to the way you drive your vehicle. I say this with the idea that most people drive faster than they should with trailers when going through mountains because they're impatient. So, they're usually trying to gain a lot of speed going uphill and go 65 or 70 plus. Shoot, a lot of times, I hear people that want to do 75 to 80 plus. I think back to when I was a tech and I was working on a few construction company's trucks and they would tell me how they would easily drive 75 or 80 plus going uphill with their trailers loaded with a skidloader or something else. They bragged about the power the powerstrokes had compared to what they used to have. Of course that 215hp/335 tq. rating is peanuts compared to more than double in today's world but that's a different story.
Point is, they drove the crap out of them and abused them. Sure, you can drive them that way and probably even go a little faster uphill with them if you want BUT, they are meant/designed to do that. Especially not on a regular basis like construction workers.
So, if you are "nice" to your truck and drive it where you maybe only lose a little bit of speed when going uphill and just try to maintain speed like around 55-60 (65 max) AND keep it out of overdrive while driving through the mountains, then you could get away with maybe hauling a little more weight a little more frequently without an additional cooler.
But this is with you keeping your foot out of the throttle going up hill to minimize the load/pressure on your trans as you're going uphill.
Also, if you change your fluid & filter every 30,000 to 40,000, you would be able to get away with a little more than if you're doing every 60,000 to 75,000. Heck, some people don't ever change it until it's at 80,000 or 100,000 and "I don't know what happened but it started slipping" comes into the shop and they have NEVER changed fluid & filter since they bought it new.
So, to me if it's a car trailer with a car on it every other weekend or so, I say you're probably ok if you drive it ok. 70 to 80 on a flat terrain is nothing and I won't get on anyone for driving them a little faster on flat terrain. It's when you're trying to race a rice burner up a hill at 80 with your camper on the back that makes me want to bi*ch slap some people. Not to mention the safety of trying to drive so fast through mountains or high hilly areas.
I'm a fast driver so I'm not trying to be hypocritical but even I know my limits of my vehicles.
Now, as for the trans cooler, one thing to **CAUTION** you about is the placement of it and how long (or big around) the lines are. Here's the thing about adding a cooler:
The transmission has an internal pump that can only pump so much fluid through the lines and inside the tranny to circulate fluid. (UNLESS YOU HAD A TRANNY BUILT WITH A HIGHER CAPACITY PUMP but let's just keep this conversation for a regular ol' trans). As such, if you add a cooler and lines, it will take more fluid to fill that cooler as well as the lines to get it to and from the tranny. As such, you will be working the pump harder to try to push more fluid through all of this exrtra stuff. So, you want to keep it to a minimum as much as possible. Make sense?
Now, all automatic transmissions have a trans cooler somewhere on the vehicle already as standard equipment from the factory. Most have them "built into" the radiator along with the cooling system for the water pump. (you will see this when the radiator has those small aluminum lines going into the top and then one coming out of the bottom.) However, trucks may have a separate cooler from the radiator (that looks like a smaller version of the radiator) either in front of, below, or somewhere close to the front of the vehicle. Some trucks do this as a standard thing from the factory and others may offer it as a "towing package" option.
I'm not sure which one yours has but you'll have to find out. Also, there may be 2 extra small radiators/coolers OTHER THAN the big radiator on your vehicle. This is because a lot of vehicles ALSO HAVE a power steering pump/line cooler somewhere near or in front of the radiator as well.
So, don't confuse your trans cooler with the power steering cooler. Usually, the trans cooler is larger than the the power steering cooler.
So, if you already have a separate cooler on your truck as standard equipment, I would just consider putting a fan on the back of the cooler (if you have room) or in front. I personally recommend putting it on the back (with airflow from the front of the cooler in front of the vehicle through the cooler to the rear of the vehicle the fan would be on the rear where air comes through) of the cooler because when you "suck" hot air out of the cooler, it cools it quicker than if you "push" it through the front. Especially on really hot days of 100 plus when you're just pushing hot air through the fins of the cooler.
But room might not allow it.
If you put one fan on and it drops your temps from 220 or 225 (not good) to 180 or 185 (more desirable and a little lower is better) while pulling a load, I would say you're good. But, if one fan isn't enough, put one on the front.
You really shouldn't need more than 1 external trans cooler (not including the one that may be "built in" to the radiator) even for most higher load and frequent hauling situations. In extreme situations you may need an additional one but REMEMBER, your pump will just be pushing more fluid through these lines and extra cooler so you may be overdoing it for your pump.
I have heard of people mounting their additional coolers under the body of the vehicle and on one side or the other but because something from the road could damage the cooler or puncture a line AND...I don't think it would cool as well since the airflow would be a lot less than the front, I think you should try to locate a place in the front if you do it. At the very least, no matter where you do it, make sure to mount it as upright as possible since angling it will decrease cooling efficiency, and out of the way where it can be damaged.
Last, I would join in the conversation on here about trans coolers if you haven't already.
Here is the link: