If it is picking up the temp of the fluid as it returns from the cooler, that would explain the difference in temp from when you are driving vs sitting in traffic. To a certain degree anyway.

 

I think the difference in temperature between the engine oil and the tranny oil has something to do with the fact that the tranny doesn't have a bunch of little explosions going on inside it every second.

 

Just one explosion in the tranny and it's done!

 

im gonna put my tranny gauge in the hot line, before the cooler, but after the rad, as seeing how that cooler don't do ****, just keep the tranny warm during the cold *** winters here, mostly b/c im lazy, and don't feel like dropping the pan to put a bung

my next truck is gonna be a stick, but im still gonna run a tranny gauge, along with an EGT, Boost, Fuel Pressure, Air Pressure, Oil Temp, and Diff temp for the rear axle, its gonna look like a cockpit inside, ill also have the CB, some switches for lights, brake controller, and a chip...godda love a diesel truck