There is no difference between "auto climb" and "auto speed". They are both alt-X. It's just that the speed defaults to the best speed for planes to climb at for whatever plane you are in. Note that this best speed is for sea level, and may not be as effective higher up. However I've not found anything better at higher altitudes (meaning have not found any other settings/speeds that produce better ROC).
Once you change the speed with a .speed xxx command, you need to know what the default was. This is easy enough. You fly a plane often enough and you'll remember what the auto climb speed is. Example, 109s hover within 5mph of 150. 190s are about 175-185.
I have tried several planes with tolerances of +/- 10-20 mph of their default speeds, with little to no noticable difference, so as long as you're in the ballpark of the default you'll still climb well.
What I'd rather have, other than a single speed, is to have the E6B tell me what my best climbing speed would be for the alt I'm at. That would rock. They MUST have had charts/hand-held computers like that to train pilots in real life.