I'm confused too - it's not just you. I'm pretty comfortable in the T-5 realm, but not the autos. I have no idea what the c4 came in or didn't - and how compatible it might be or not. One thing that does make sense to me is this - hook the speedo up with the gear that's on it - if it doesn't have one on it, buy one with a big number (21 or 23 teeth). Then compare your readings to mile markers on the Istate (odometer) and to timed runs between mile markers (speedometer). Then you'll know how far off you actually are. At that point we may be able to suggest alternatives. My best guess is that they don't make enough teeth on the driven gear to correct yours - and you'll have to either 1) put in a speedo box, or 2) replace the drive gear in the tranny with one that has fewer teeth (drop from 8 to 7, or 7 to 6), or 3) put taller tires on the car to help. But I don't think anyone can be specific about any of that unless/until we know how far off it is with a particular gear in it to start with.