Joined: Jul 2002
Posts: 40,863
Likes: 3,116
From: Sacramento, California, USA
Bikes: Specialized Tarmac, Canyon Exceed, Specialized Transition, Ellsworth Roots, Ridley Excalibur
Enough tension so that the tire doesn't slip. If it slightly indents the tire, that's good enough for me.
You can use whatever gear you want on the trainer.
You are not going 18mph on the trainer. Your speed is zero. Track time and effort (power, heart rate, or good old fashioned rate of perceived effort).