I had this problem on my '04 Ultra Violet in November last year. I'm not sure if my husband asked about it or not but there might be more info to find. We were going to SEMA in Vegas and my odometer at one point was reading 75 and EVERYONE was flying by us. I had a navigation system and we checked the speed on it and I was going 48mph. It was a real blast when my odo showed we were going 148mph ( actually 80). We went to the dealership in Vegas and he said they were having problems with it, they new about it and it would take 8 days to get the part to fix it from CAlifornia. - I told him I was from Cali and I would get if fixed when I got home.
Strange thing, after I talked to the dealership, it never happened again!? I think the Ultrviolet is possessed.

- Brat