Abstract: A vehicle road speed control system includes a vehicle road speed detection means, parameter storage means, comparator processing means and control signal generation means. The system is arranged to compare measured vehicle road speed with predetermined road speeds obtained from stored parameters. When an upper threshold road speed is exceeded a control signal is generated for an associated transmission control system which causes the operative ratio of an associated vehicle transmission to be reduced such that the vehicle road speed is reduced to at least the upper threshold road speed. The road speed may also be controlled by altering the characteristics of an associated fuel injection such that the road speed is reduced to at least the upper threshold road speed. This may be done in place of controlling the operative ratio of the transmission or in addition.