Most cars today have onboard networks, where the predominate network
protocol is CAN. I want an equation to calculate CAN busload in
percent from the number of messages, average message length (in bits),
bus speed (kHz) and average cyclic transmission rate (in
milliseconds).

Failing this I would like a standard equation to calculate busload on
a serial bus. Where I want busload in percent from the number of
messages, average message length (in bits), bus speed (kHz) and
average cyclic transmission rate (in milliseconds).