I'm not 100% sure that I understand the "context" of your question regarding the accuracy of the speedo in relation to the Wheel/Rim & Tire diameter, but I'm going to take a "stab at it" based upon what I think your asking.
I'm going to have to disagree! I finally think I am close to what might be considered by most a reasonable suspension fix for about $2,100.00. I have the QA1 shocks, (12 way adjustable) with the Eibach springs on the back and the stock spring, (1/3 coil) cut on the front. After some several different setups, 1=3/8" spacer added to the front shock. I have the front set at 9 clicks and the back at 8 clicks, no fender rub, nice ride, better than stock handling which was my ultimate goal. Although I do agree the Hotchkis sway bars go on next. Oh can't forget the Kars stage 3 cradle to eliminate the wheel hop and improve the rear end handling. Basically makes the cradle solid to the body of the car. No reduction in ride comfort noticeable. If you check out the Team Cadillac cars you will see that the have basically (welded the rear cradle to the chassis).
All that said the total cost so far was $1,700.00. Add $400.00 for the Hotchkis swaybars and it's done.
FYI, Frank, after you left yesterday I added the spacer and cranked the rear two clicks. Much nicer!:bouncy:
I do have one question, how can you close the wheel
gap while maintaining an accurate speedo with rims and tires only? If you change the circumference of the
wheel/tire you change the speedo. If you don't change the circumfrance you don't change the wheel gap.
Remember that regardless of what method you use to lower the vehicle; whether it be lowering springs or adjustable coilovers to close the wheel gap for aesthetic purposes or perhaps you are truly from an engineering standpoint taking into account every facet of the suspension system with regard to suspension geometry; and if that is the case then there is one thing that is CONSTANT. And that is the FIXED position of the lower A-arm, spindle and hub.
That axis position does not change unless you are installing a drop spindle. That being said; the lower A-arms, spindles and mounting hubs remain in a relatively fixed position regardless of the method that you use to lower the vehicle, ( lowering springs, adjustable shocks etc).
That being said, the factory speedo is calibrated based on total Wheel diameter in relation to RPM's. Currently, the speedo is calibrated based on an 18"x8.5" wheel rim with 45 series rubber. For argument sake, lets just say that the total OEM Wheel Rim including the tire is at a diameter of 27". When you increase the diameter of the rim size from 18" to 19", there has to be a corresponding change in tire series size (say 40 series) to maintain the same number of RPM's to keep the speedo accurate as originally calibrated.
To complicate things even further, when you increase the wheel rim depth from the OEM 8.5" to something more aggressive like 9.5" in the rear, you would have to compensate with 35 series rear rubber as you noqhave to account for rear tracking which comes into play with regard to the suspension geometry in keeping the speedo calibration accurate with regard to the thrust angle.
I'm not sure if that answered your question, but lowering the vehicle via springs and shocks and the fixed position of the wheel spindles and lower A-arms with regard to alterations in Wheel/Rim and tire size diameter are mutually exclusive with regard to speedo calibration.
Also, like you; I'm a huge fan of saving money on performance modifications, but my only concern with the QA1 shock and spring set-up is what kind of warranty or assurance do you have from the manufacturer that you have not compromised the front and rear suspension geometry and it's components with this revised shock and coil-cut set-up?
Only time will tell if you begin noticing negative camber issues, uneven tire wear as well as premature wear of the various suspension components (trailing arms, toe rods, spherical bearing, etc) of your vehicle. There is also the concern of front end scrub radius issues with cutting the coil springs to fit. I would make absolutely sure that you have adequate clearance between the upper A-Arm pinch bolts and front tire sidewalls as you could very well run the risk of a tire blowout under highspeed.
Just a thought on my part as I know that allot of folks have asked QA1 about producing a true "bolt on" adjustable shock and coil-over package for the CTS-V, but they have yet to step up to the plate with a "finished and warranted product" and you had to resort to building your own. It doe sounds like you put allot of thought into it beforehand and QA1 does manufacture quality products and spherical bearings. It is just the application engineering that would have me concerned without any engineer testing or CAD modeling.
Granted $1700 and your personal labor is allot less than the $3250 that Mallet wants for the Penske set-up. But at least with Mallet that extra $1500 buys you a 2 year 24,000 mile warranty which could possibly prove to be far less in cost should you run into any premature suspension wear problems down the road. Just my own opinion, but I do understand the value of saving money up-front, just as long as you don't end up paying more down the road to fix what wasn't broken to begin with.
You should post some pictures of your suspension modifications as it does sound as though you put allot of thought into it beforehand and I'm sure others would like to see the results of your efforts.
Best of luck with your QA1 modifications and perhaps your onto something that no one else has given any forethought to. We certainly know that QA1 has not as of yet!
Please share some pictures and instructions of your efforts and what you have done as there may be others that have ideas to share that could be of mutual benefit to all ***********; especially at $1700!! QA1 may end up owing you a royalty if your onto something!
best regards - Pete Raimondi
Cadillac MotorSports, Ltd.