Hi Rob, this is not correct, unfortunately you were given dud info.
The only reason this is not correct is because Toyota do not specify a GCM. Any changes to the GVM are therefore independent of the GCM, and thus towing capacity. (This as per ADR rules/legislation, ie Law).
However, if someone is providing a 'Claytons GVM upgrade' that specifies a derated towing capacity (or a GCM that derates the towing capacity) - walk away Kev! This is wasted money.
GVM upgrades for LC200's are very popular down south for the grey nomads with big vans - and they retain full 3.5t towing. Check out the caravan forums.
Cheers
Brendon
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