When buying a new transmission, try to purchase the one with the lowest possible mileage, either by checking the odometer of the vehicle the transmission came from or by estimating based on the year of its production. Perform a comprehensive visual inspection to make sure the transmission is not damaged, as well.
If attempting to estimate the mileage of a used transmission knowing only the year of its manufacture, assume that each year accounts for 13,000 miles. If the transmission comes with no such record of manufacture, it is not recommended. It is worthwhile to request a vehicle report using the VIN number of the original vehicle, when possible.
When performing a visual inspection, search for signs of fluid leaks on the seams and gasket seals. Also, look for cracks in the transmission case, and physically manipulate the shifter linkage, going through the gears and feeling for shifting gaps or other problems.
When looking to purchase a used transmission for a new vehicle, pay extra attention to the exact year and model of the car the transmission comes from. Many modern transmissions make use of computer sensors and other systems that are unique to the specific model for which they were designed. Even a small discrepancy in the model, year or make of a car's transmission can invalidate any benefits.