Odometer problem

Hi -

I'm having a problem with the odometer/speedometer in my 2003 E350 van. I bought the van a few weeks ago, with 22000 miles on it. It seemed pretty zippy to me but i attributed that to the 6.8 v10, quite a step up for me in size from a v6. Turns out it seemed so zippy because the odometer is reading about 10% high and the speedometer about that as well. I've compared the odometer to the mile markers on the highway and it's consistently measuring 1.1 mile for every 1 actually driven. The speedometer is about around 10% high as well. I'll look at it on the highway and see 85-90 mph and see traffic passing me all around.

I took it back to the dealership i bought it from and they can't figure it out. It still has the original tires and they say they checked the speed to the speedometer and it's within Ford specs. I took it to a shop and had it checked using a dynameter (think that's what it's called) and it read about 10% high, worse at faster speeds. So i took it to another dealership (one i think of as more competent than the first) and they can't find anything either and said the speed measured at the front and rear are consistent with what the speedometer says.

Does anyone have any ideas or experience with something similar? This is making me crazy, not knowing how fast i'm actually going. And, even worse, i'm driving out my factory warranty and the expensive extended warranty i bought at a 10% faster rate than i should be. And i drive enough miles as it is without adding an extra 10% on top.

Thanks a bunch for any ideas or suggestions!

-Robin