  We study the dominant non-universal contributions to muon decay for the five
scalar leptoquarks (LQs) allowed by the Standard Model gauge symmetries. By
using a common computational framework, we isolate the new-Physics (NP)
one-loop corrections over the observable $\Delta r$ and explore the connection
with the $W$ boson mass. We highlight the role of non-universal terms
considering, for the $SU(2)$ doublet and triplet LQs, a completely degenerate
scenario, thus cancelling/moderating the effects of the oblique parameters S,T
and U . We determine simple formulas that exhibit the leading non-universal
behaviour generated by the introduction of extra Yukawa terms, and confront
them against a present tension in $M_W$ and the projected FCC-ee accuracy.
