Can you hear the difference in R2R format

I am wondering if you can hear the difference in tape speeds 3 3/4 and 7 1/2, in most recordings.
yes, night and day difference!
depending on the tape and the music recorded, yes you can hear it. It is quite obvious on my system.
If you play a 7 1/2 ips at 3 3/4, you'll definitely hear a difference. Bada bing!

Okay, now that I've gotten that out of the way for all who follow, if high fidelity is your quest I wouldn't even bother with 3 3/4 ips tapes. Which causes me no end of head-shaking when I see the prices that some 'pop' commercial tapes go for. I can only assume that the buyers are discography completists, uninterested in actually playing the tapes. Or at least, that's what I'm hoping.
I had a tape deck that had much better 3-3/4 ips performance than it typical. It was a Tandberg, and it was linear out to 20,400 Hz. I often used 3-3/4 ips to record albums, but this was in the '70s, and I didn't extract nearly the dynamic range from LPs that I do now.

However, if I was dubbing from a truly high end recorded source, and that includes the few classical Advent Cr02 cassettes produced in the '70s (or direct-to-disc LPs), I needed 7-1/2 ips. And if I was recording live, nothing less than 7-1/2 ips would do, and even then I tended to use Tandberg's peak limiter.

Furthermore, true usability at 3-3/4 ips also depends on the quality of tape. It took a top end premium tape from TDK, Maxell, or 3M to be able to use 3-3/4 ips without hearing dropouts from tape oxide flaws.
Tape age and condition can be a major factor today, adding to the difference in the quality of the sound. From buying 'lots' of commerical recorded tape, I sometimes get duplicates and often when I play each, different quality of sound, that goes for three's and seven's. So, condition is a big issue today. I don't do any recording, just playback. Since I have more then one deck, playback is also affected by the alignment of the heads. Still, some of my tapes sound better then the vinyl and cd, you figure.
As a general rule, I won't buy any reels on Ebay that are recorded at the slow speed. That said, I do have a few slow-speed tapes which don't sound half bad. It's pretty much a crapshoot in any event, since all the tapes available today (with the exception of Tape Project reels and a couple of others) are at least 25 years old and heaven knows how they were treated.