A system and method establishes an acceleration of a vehicle which may be used to control a brake system of a towed vehicle towed by a towing vehicle. The system and method establish a gravity vector representing acceleration due to gravity, measure acceleration of the vehicle in a first direction and responsively establish a first acceleration value, measure acceleration of the vehicle in a second direction and responsively establish a second acceleration value, and establish a magnitude of the acceleration of the vehicle in a plane orthogonal to the gravity vector as a function of the gravity vector and the first and second acceleration values.