Change-up to Fastball Speed Differential

What do you think is the best speed differential between these two pitches to get batters out at the college level? Mine has generally been around 15 mph slower than my fastball but my college coach is strongly encouraging us to get our change AT LEAST 20 mph slower than our fastball. Iv also heard that 8-10mph difference is optimal too. The change-up is definitely essential to a good pitching arsenal, how do you guys feel on this issue?

I think the faster your fastball, the less of a differential you need. 20mph seems like a lot to ask if you’re going to maximize your deception by throwing your change with fastball mechanics, arm slot, arm speed, etc.

You have to throw every pitch with the same arm motion and the same arm speed—one thing you do NOT want to do is telegraph your pitches. You can change the speed of various pitches by tightening or loosening the grip, because therein lies the deception which is such an integral part of the changeup. A speed differential of 8 to 12 miles an hour is, I believe, sufficient. Most major league pitchers do this.
That is, of course, assuming the fast ball. Now, there are a number of pitchers who don’t have one—they are the finesse pitchers, the so-called “junkballers” who don’t have much in the way of speed and so have to rely on an arsenal of breaking pitches and the control and command that go along with them. They have to resort to other means.
For example, look at Jamie Moyer. He’s the latest in a long line of finesse pitchers who throw the breaking stuff—and he does very well with it. In fact, some of these pitchers are even deadlier than the rip-roarin’ fireballers. 8)