I have a 17 mountain mile TT in September that progresses slightly in steepness 4-7% gradient. I would appreciate some strategt tips on gearing.

Should I ride a 39x21 at the start and than gear down to a 39x23/25 as it gets steeper (and I tire) or should I start out in my lowest gear from the beginning and just spin my arse off to keep my legs fresh for when the road does turn upward?