My SM is a 2004 with a Yanmar 4JH3-HTE engine. It does have a Hurth ZF25 gearbox. However, it may not be the same gearbox on your SM with a Perkins M80T.
In any case the ratios on my gearbox are as follows:
Ratio 1: A = 2.80 B = 2.80
I'm not sure what you are going to use the ratio information for, but the only way to be 100% certain what is on your boat is to look at the plate that is on the gearbox in your engine room. Maybe you can get someone to go on your boat and take a picture for you. Here is the plate on my gearbox:
I find that Google Photos is a great way to store information like this "for posterity" so that you can find and access the information even when you are not on your boat. That's what I just used to grab the photo above. I took that pic over a year ago for no real particular reason but to document what I had while I was replacing the Vetus coupler.
SM #440 Cara
Deale, MD USA