cekim

Those are Fords code for spring rates... So what they mean is that the 18" (stiffer, lower profile wheels) have softer springs ("G")

So, in theory to go with 20's and a 35/40 profile, you would want even softer springs (somthing higher in the alphabet than "G") as Ford's thinking goes...

Bottom line, I don't think it really matters to you if you are willing to deal with 20's and a 35/40 profile tire (read DAMN HARD), then I don't think you care and if you are lowering the car - good luck on finding steeda/roush/FRPP/Saleen springs that give a hoot what profile tire you are going to run...

You get "sport-line" or "pro-kit" from steeda - not 12 variations....

Springs from other places don't follow the same scheme and in fact few publish their spring rates, so the best you can do is ask someone with your car and the springs you want for a ride to see if it is too rough...

marlinmon07

cekim is right. even saleen suggests 35/40 aspect ratio on their 20" rims. overall, i dont think we are talking about a massive performance difference here

cekim

It is a little surprising that ford went to the trouble... Indicates that it might matter more than I would have thought - at least to someone...

From what I have heard in various places Ford went to a number of extremes for "cockpit noise" - I would guess that is the likely suspect (sheesh they used a !@#$ing 2 piece driveshaft for that reason from what I have heard) - but that is all hearsay... I know nothing...

With the top down, tailpipe singing, and SC whining, I can't hear "interior road noise" and even if I could, I would like it...

The only reason I keep the side glass on any mustang is to button up the car when I park - otherwise they are just dead weight