When I used the VAAS USB Cassette Capture device and Audacity software to create a aud or mp3 file I would get a file with about 104 minutes from a C90. I was concerned that the files were bigger than they should be. I thought most of my C90s had approximately 94 minutes on them. Even though the files I was getting were about 10% longer than the originals they sounded normal. Then the device stopped working because I dropped it.
The replacement device Super USB Cassette Capture by ezcap seems to have a problem with speed in the other direction.
One tape I have (Grease) shows on the Cassette info side one 32:09 and side two 31:30 for a total of 63:39. Using the first device I got 67:51 and with the second device 59:15. For a C90 that should have about 94 minutes I was getting about 104 minutes with the first device and 88 minutes with the second. The sound seems normal from either device. I am just trying to understand tape speed.