Now my speedo is ~10mph off (compared to my GPS). I've taken the needle off a couple times and set it at zero when idling but still it won't hit the right number. Any ideas??

I thought so too! In fact I was expecting bulbs when I pulled the gauges. I have no idea why some (2 of the LEDs out of 6 on the board) quit. I was thinking they weren't getting power but that wasn't it. Strange for sure. Of course had it been any of them other than the spedo I wouldn't have messed ...

My 2005 S40 speedometer LEDs quit working. These are little LEDs that are soldered directly to the board that controls all gauges. There are several posts on how to get the gauge cluster out. Once you do that you can take the whole module apart (with just a phillips screw driver) to get to the LEDs ...