If it is just a standard vehicle and not a performance vehicle, (performance being custom built or specialized engine in some way.) You want an OE temp/spec thermostat. Prices range generally from $8-$30 depending on the vehicle. Call the store and they can help you get the right one.
It's not the size that really matters bc in each vehicle it's own same size rather the temperature rating is what you need. That depends on how hot you want your heat and how soon you want the thermostat to release the water for cooling. I suggest you call the store or go to the website to view the selection available to you for your specific model vehicle.
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