Unless you had a very very very very long hill to ride down, I doubt a 583 would even pull 15/36 gearing, it's just way too tall (in 5th gear).
If you did find a very very very very long hill to ride down, then you may see 140mph ish on that gearing.
Think of it like gears on a push bike, if you select the biggest sprocket on the front and smallest on the back, it's really hard to pedal, but once you get it wound up, you go quicker (and pedal less)
If you select the smallest front gear, and biggest rear, you pedal like mad and don't really get anywhere.
Hope that helps
