I was never happy with the progressive springs, always too harsh and wouldn't let the shock work! With a 650 pound wheel weight, the 250# springs is way too harsh IMO. As for the Speedway chart I too found it very inaccurate and Seth's comment on the shocks is worth looking into also. If you can make it through the salesmen on the phones at Speedway and actually talk to one of the AFCO shock and spring techs you should get an accurate answer to the correct spring rating. I have a leftover pair of 200# springs that I took off a guys '34 coupe and went to a 150" AFCO spring, he hasn't been back so I guess they were ok! LOL, of course, your results may vary! Again, just my opinion but I believe the double adjustable shocks are worth the extra $$$$ just for the ability to tune them!