a gear from drivetrain to odometer? lol, definitely not.
and the answer is no. The ECU does contain the mileage, but anyone who has enough skill to replace a cluster can also fake the mileage in the ECU.
People who reprogram mileage on cars can do it so well that the dealer cannot tell its been done (so you can return over-mileage lease cars without penalty)
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Hot: 2001 Estoril M Roadster
ZCP 19's michelin supersports, ZHP knob, JL 8W3
Cold: 2002 TiAg M3 6mt deoranged, dechromed, led tails, ZHP knob, UUC SS v3, GROM, OEM 18's w310's,
Fun: 2006 YZF-R6, black, ohlins, pazzo, cat delete, vance hines csone, YEC racing tune