Swing speed vs ball speed

Kinda new here and had a question. I just traded in the driver I had been using (Taylormade Superfast 2.0 with regular flex) and got fitted for a new Cobra Amp Cell with stiff flex. The guy doing my fitting at Golf Galaxy mentioned that with my swing speed of 104 (rough average) he would want to see a higher ball speed. My ball speed averages around 140 and he allowed that 10 mph faster would be more normal. Disclaimer: I travel so I wasn't truly prepared and had on my usual cowboy boots instead of tennis shoes. The wife gave permission for a new toy so I took the opportunity when I had it. My question is how can I change that? Is it simply a matter of not hitting the sweet spot or am I doing something else wrong? I apologize for the wordy post but would appreciate any insight y'all might have. If I need to provide any additional details let me know. Thanks.

The theoretical maximum (given the USGA rules governing driver construction) "smash factor" is 1.50. So a 104 mph swing speed could theoretically produce 156 mph of ball speed with perfect contact. The most common factors that will lower your smash factor and hence ball speed are missing the sweet spot (particularly towards the heel) and/or a "down-and-across" club path that results in more "glancing" contact with the ball. Get a dry erase marker and color in the face of your driver (don't worry it comes off with a wet wipe) so you can see where your impact location is. If you are consistently hitting it off center, try choking down on the shaft a few inches. Modern driver shafts are generally too long for average golfers.

Ball speed is a function of swing speed, quality of contact, and the properties of the clubhead and ball. With a swing speed of 104, ball speed around 140 isn't too far off. Generally speaking, the maximum "smash factor" is 1.5, meaning a well struck drive should have a ball speed about 1.5 times the swing speed. So in your case, a perfectly struck drive would have a ball speed around 155.

I just had a fitting a couple weeks ago, and I started with the same driver! When I did my normal driver swing, he said that I was holding back and swinging too slow (as compared to my irons). He thought that the regular flex TM driver might have been erratic as I sped up my swing, so I self-regulated it to the 90s. When I tried other drivers, I started to swing at a faster speed for some reason... maybe the feel of them?  My max was 103, but most were low 100-102.  I hit about 5-10 different drivers with various differences, allowing the TrackMan guy to zero in on what he thought was best (they all felt decent to me). He settled on:

Titleist D2 loft 7.5, A1 setting, stiff shaft, 45"

My overall recollection of the stats for my drives was a ball speed around 150mph, when I hit 100-103mph. The "smash factor" was typically 1.47-1.49. He didn't worry about distance because we were hitting into a stiff headwind, but they were longer carries than I was hitting with my TM, that's for certain.

Anyway, I only posted, because I had a recent TrackMan experience and my parameters were nearly identical to you, and thought you'd be interested. I wish I had the $399 for their driver recommendation, but I doubt I'll buy anything quite yet. What's interesting is that the whole experience gave me more confidence to swing harder with my current TM driver, and I am hitting more confidently. But it's good to know that I'd likely benefit from stiff shafts when I'm ready to make the plunge.

Another easy way to check driver impact is to use masking tape on the face.

Thanks for the responses.  I didn't know about the smash factor and numbers and all that - good to know.

There's "normal" and "optimal". As said above, balls and clubs are limited by regulation and design, so that a ballspeed of 1.5 x clubhead speed is a maximally efficient strike. Impact efficiency is bound to be a function of your ballstriking efficiency - so I don't see how "normal" can have any real meaning without a reference to your current playing ability. I don't have empirical data, but 140mph ballspeed might be better than average, or worse than average, for other players of your standard - and still have little or nothing to say about how well your current driver fits you.

Face tape, or ink, or talc to record where on your face you impact the ball is interesting - but it doesn't tell you the whole story. What matters is how the clubhead CoG and the centre of the ball align through impact. The clubhead's CoG is set far enough back from the face that I believe the point of impact recorded on the face could, depending on the face and path alignments, give a misleading impression of how "centered" your impact really was.