Land vehicle speed is usually measured by wheel speed or GPS. While these methods are adequate for some purposes, there are some drawbacks. Wheel speed may differ greatly from vehicle speed due to tyre slip. In addition, speed measured by GPS contains little high frequency information and lags the actual vehicle speed. A method is needed which combines both accuracy and good transient behaviour. This paper describes a method that combines GPS and a Reduced Inertial Sensor System (RISS), in this case a single accelerometer, to achieve an accurate estimate of vehicle speed. Using a Kalman filter, the low frequency accuracy of the GPS and high frequency response of the accelerometer are combined. A number of error correction strategies are applied to provide a robust and accurate measurement system.