Well i am gonna go the other way and say i think it IS possible. (that your friends saw 170km/h on the speedo).
Heres what makes me think its possible for the speedo to read 170-
i. speedo error
ii. do they have aftermarket tires which further increase speedo error?
iii. it was a downhill stretch (even if not noticable..it still helps)
iv. as ajmat said...a tailwind would definitely help as well
cya
R
ps - does the xing have diff gear ratios than the zip?
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