On an interstate going 65mph? Did they own cars with 4 speed automatic transmissions? lol.

 

I have to climb pretty steep hill near my house and with other cars I've owned with small engines I always had to downshift to prevent lugging the engine. I have never had to downshift with my S except when I'm behind someone going really slow.

If you're at 65 mph your RPMs are close to 4000 in 6th gear. You have plenty of torque at 4000 rpm. But if you choose to pass someone when going 55 up hill, then it would be wise to downshift, like with any other car with a manual tranny.