Once you have a known weight value, and you can measure what is in the car now, use this link and on the left side is says "Select your vehicle", with the known info you have you can review what AFCO suggests and compare what you have and choose a spring that more closely matches what you're trying to do.

https://www.speedwaymotors.com/shop/...25-16-306-7512

having the height, spring thickness (of a single coil) and ID / OD will get you closer to a spring your looking for. And you may have to disassemble a spring/shook to get a true height. Usually the thickness of the wire used to make the spring will help define the weight rating. And I found that for me the AFCO selector gives a spring that is to heavy, I like a lighter rating to get a smoother ride.

Hope it helps.