Hi Skurcey, welcome to Subsim!
As said above, the stadimeter can give erroneous readings. And as you said, wrong speed can make massive differences. In fact I'd go so far to say that speed is one of the most important readings to get right, too slow and your torp slips behind, too fast and goes way infront of the target.
Have a look in
HERE - Should give you some handy tips!
Sev.