I was planning on lowering my car with just springs. right now I have 18's on it
and it looks a little high. Which springs should I get? I don't want to go TOO low.

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https://www.scionlife.com/forums/viewtopic.php?t=89766

please use the search button next time as this is an announcement in the suspension section...which is at the top of the page of the suspension forum that you posted in.
Anyway, this post i linked above has a chart showing all the heights and spring rates of coilovers, shocks, and springs

 

which springs are most reliable?

 

they're all the same in essence...it's the shocks that you need to worry about...the lower the car, the more force gets put on the shocks

 

they're springs, they're coils of steel, as long as it's name-brand you can't go wrong, you just need to find whatever drop you want to go with.

the diameter off the wheels has no effect on what springs you can or cannot use.

 

All depends on how low you wanna go, I wanna be real low, went with the Tein S-Techs, 2.4(front)/2.0(rear)