TriBlk....I've been reading your post about SpeedCal for a while, and have another question to ask you. Since you've done the math about the gear corrections.
I notice when you plug in the stock 3.27 rear, around 810-815 tire RPM, 7 tooth drive gear , from stock Cobra setup (96-98), in the Ford formula, you get a number like 18.5-18.6 teeth, and yet Ford uses the 19 tooth speedo gear. Most mags said the bone stock 98 Cobra speedometer was very accurate. If that is so, then when you change rear gears, one needs to go with a tire/wheel combo that nets out a number a little under the 20 (for 3.55), or little under 21 (for 3.73), or little under 23 (for 4.10). Or use a correction device like yours.
As an example, from 3.27's to 4.10's is a little over 25% change, but from 19 to 23 teeth is a little over 21% change. That 4% difference in change (to get an accurate speedometer) has to be made up for with a larger tire circumference, a correction device like yours, or just be aware your speedo is off 4% with stock 25.7" tires. A lot of us are using 25.9-26.1" circumference tires now anyway, which takes up about half of the 4%, so we are a little closer to being accurate. I'm not so picky about speedo accuracy that I worry much about being 2-3% off, but I wonder if I am figuring this correctly. Of course, I need to replace my 23 tooth for sake of durability. Thanks for your responses.