That's odd.Race Tech has a spring rate chart on their site.Just enter the info.A Zilla spring may work,though its a fair bit longer and I,d have to check if its small enough on the one end.The 250,s are tapered not sure about the 500,s.Like I said the rear springs are too soft.Gonna check on the shipping tomorrow but if it was me I,d go with a stiffer spring.