You cannot determine the correct amount without knowing the transmission type (e.g., AX-4N, M5OD-R1, etc.) and consulting your owner's manual or a reliable repair manual specific to your truck's year, engine, and transmission.
Do not attempt to add fluid without this information. Overfilling automatic transmission fluid can cause serious damage. Checking the transmission fluid level correctly also requires a specific procedure (usually with the engine running and transmission at operating temperature for automatics).
To find the correct amount:
1. Locate your owner's manual: This is the best source. The transmission type and fluid capacity should be clearly stated.
2. Find your vehicle's identification plate: This often lists the transmission type.
3. Consult a repair manual: A Haynes or Chilton manual for your specific year and model Ranger will provide detailed information, including fluid capacities and diagrams.
4. Check online resources: Websites specializing in automotive repair information might have this data. Be cautious and only use reputable sources.
Once you know the transmission type, searching online for "[Your Transmission Type] fluid capacity" will likely yield the answer.
