Honda 2005 VTX1800C Owner's Manual page 121

Coolant
Decreasing the concentration of antifreeze
to less than 40% will not provide proper
corrosion protection.
Increasing the concentration of antifreeze
is not recommended because it decreases
cooling system performance. Higher
concentrations of antifreeze (up to 60%)
should only be used to provide additional
protection against freezing. Check the
cooling system frequently during freezing
weather.
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Servicing Your Honda