In various example embodiments, a system and method for determining a trailer brake gain signal for trailer brakes on a trailer being towed by a vehicle, and applying brakes to the trailer is disclosed. A method includes: providing predetermined calibration settings relating motor drive force to motor speed for a vehicle travelling at various speeds and providing accelerometer data. The method then determines that one or more vehicle performance parameters fall within threshold ranges and then determines both the vehicle weight and the trailer weight. The brake gain signal is determined based on the ratio of the current trailer weight and the original trailer weight. The brake gain signal is then transmitted to a trailer brake controller that applies trailer brakes according to the brake gain signal.