Actually. you may not want the largest cooler you can buy. You do not want to cool it too much. If you cool it below operating temps, It will not operate correctly. In fact mine will not even go into overdrive until it warms to the proper temp.
Best to size it to the vehicle requirements, and in truth. the factory towing package cooler is plenty good.
You cant cool the trans enough. I have a gvw 20,000 on my s10 p/u. My temps towing stay at 170.. Sometimes as high as 190 when hitting a hill and I come out of OD or the TC unlocks.
In the winter I put a piece of cardboard in front of it...
If you look at your stock cooler there is no way its sized properly at all.
Here is whats recommended..
http://www.bulkpart.com/transmission-cooler.html\
If you go to the auto store and look at a gvw 26000 cooler then compare it to your tiny stock one, you will be amazed...
trans temps guide here.
