I think you'd know if you were going that fast...because you'd be completely spun out at that point. The largest "normal" gear combo I can think of is a 53x11. At 45mph, you'd be spinning at 120rpm.

The best I have is a 52x11 and I've only had it up to the low 40's...and that was plenty fast enough for me. At that point, I was spun out, I can't do 120+ rpm in a smooth enough manner to be safe doing it.

My Strava phone ap once told me I hit over 50 on a downhill while in a large group. Looking at other data from riders in the same group that actually had Garmin units, their speed was only in the mid 40s. That drove me to get a Garmin myself...