Hello,

I have done just that but be careful, the reels should be changed, so
remove the leader from the empty reel, install on the bigger, insert the
reels into your big tape, full reel still small and empty is big, then
very carefully keeping the tape horizontal insert into vcr, spool all
the way towards the end and replace other reel, then rewind.
The reason is that the small reel can fall out of the hole and jam the
tape (or your deck!).
I would just buy the adapter, not worth the hassle unless it's an
emergency (which was the case when i did it).
But no worries about speed differences, that won't happen.

Cheetah