Can you define really hot?

My truck was radared at 42mph but geared for 33mph FWIW. So with gearing for 37mph(about the same as my current setup) I expect it's doing into the mid to upper 40's. A quick calc say's I should be close to 47mph with my setup, and I would say it feels close to that. My motor is getting up to mid 140's which does feel quite hot to the touch, but perfectly acceptable.