On rear wheel drive vehicles the speed sensor may be found on the output shaft of the transmission or located within the rear differential. (yourmechanic.com)
This ratio can not be changed on most front wheel drive vehicles, but can be changed on many rear wheel drive vehicles. (yourmechanic.com)
This however does not hold true for rear wheel drive vehicles which can have a strong weight transfer (high, soft - suspension cars, as most US vehicles), as the added braking force on the rear wheels without added (weight - transfer caused) friction could cause wheel lockup and possibly loss of control of the vehicle. (mechanics.stackexchange.com)