I have a 95 F-350 Powerstroke. The speedometer works up until about 30 mph, then it just goes crazy bouncing all over. I changed the vss and checked the wiring on it. It didn't help. So I decided to change the oil in the rearend. The guy I bought it from said everything worked on it when he parked it. It sat for well over a year though. When I took the diff cover off the tone ring had about a 4 inch strech of bad flaky rust on it. To the point that you couldn't see the teeth on it. I am guessing that there was moisture in the rear end and it settled out while it was sitting. I thought well there is my problem. So I took my die grinder and wire wheeled the rust off and got it clean and shiny. The teeth are badly pitted and miss shapen now from the rust. The speedometer still doesn't work. I think that it is most likely the bad spot in the ring causing the problem. The mechanic I talked to says it can't be the ring or the check engine light would be on and several other problems would be going on. He says it is my speedometer head. I have an appointment to get it scanned and checked on Tuesday. I was just wondering if anyone else had had a bad tone ring or if it was a common problem.