I got my 4.10's put on this weekend and changed my speedo gear out as well. I knew going into the install that because I have an automatic, no speedo gear will calibrate the speedometer perfectly. Anyways, I put my 23 tooth gear in and then got out my portable GPS unit and wanted to calculate how far off my speedo was. Not knowing for sure that if my car was dead on to begin with (white face gauge installation with removal of needles and 275/40's out back), I found that I have about 4-5 percent error. At 100 mph on the speedo, the GPS unit reported a speed of 96.2 mph, and at 60 on my speedo, the GPS said 57. Anyways, I just wondered if others found there was a similar error when they did their gear swaps and what tire size (205, 245, 315) would make up for this error if that were possible to cancel out. I will make sure I post timeslips when I (hopefully) go to the track this weekend!