Well, it's not that simple though. You have 2x2 possibilities:
1. unaspirated unvoiced (av)
2. unaspirated voiced (aV)
3. aspirated unvoiced (Av)
4. aspirated voiced (AV).
German and I think English have 2/aV (b, d, g) and 3/Av (p, t, k), the diagonal so to speak (for the beginning of a syllable).
Mandarin has 1/av and 3/Av. So, 3/Av is pronounced the same - and Pinyin renders those as (p, t, k), as in German/English, so that's actually nice. Wade Giles renders them as (p', t', k'), indicating the aspiration.
German and English do not have 1/av (at the beginning of a syllable). Pinyin then chose to render them as (b, d, g), correctly indicating the unaspirated character, but tempting German/English etc. speakers to voice them (wrongly). Wade-Giles render them as (p, t, k), tempting German/English speakers to aspirate them (wrongly).
No easy solution. Plus, the apostrophes are so often ignored (e.g. Pinyin Taibei = Wade-Giles T'aipei).