To be absolutely certain, you should:
* Check your owner's manual: This is the most reliable source. The manual will specify the recommended battery group size.
* Look at the battery itself: The group size is usually printed on a label on top of the battery.
* Check the battery tray: The tray itself might have a molded-in indication of the correct group size.
* Consult a local auto parts store: Provide them with your vehicle's year, make, and model, and they can look up the correct battery for you.
While Group 51R is the most likely answer, confirming with one of the methods above will prevent buying the wrong battery.
