"Speed" could be any stimulant, that's why. It's also usually heavily cut (with e.g. water, glucose, chalk), and in some cases might be a wierd ass drug like a cathinone (MDPV/buphedrone/methylone) or even methipropamine...
For amphetamine sulphate, try about 30-50mg.
For methamphetamine salts, try 10-30mg.
Usually amphetamine lasts 6-12 hours, and no taking more won't neccesarily change the effect, just make you more tweaked out.
+/- 10mg scales are uaully good enough. A +/- 1mg scale is better.