Well my truck does what I see the majority of other Tacoma trucks do, the speed reads a little high (by the way almost all cars do) by 1-2 MPH over the 25-70 MPH range. The Odometer is always the same, reads .1 miles less than actual mileage for just under a 10 mile strech.
What does this mean:
1) Your ODO will reflect less mileage than you actually drive on new tires but as your tread wears I suspect it will be close to actuale miles driven.
2) Like so many other cars on the road, the speedometer does not read true speed, most actually say you are going faster than you are. I don't know of a mechanical reason this occurs unless they factor in you could use a larger tire, but I doubt that. I like to think it's a way to keep you under the speed limit, even when you think you are speeding.
I think we answered the thread starters question.
For the few of you saying you have the opposite problems, I strongly suggest you use a different GPS and double check your facts. If you have a problem you should clearly contact the dealership about it.
A little history... Honda had a problem in thier 2002 Accord (could be more but that was all that affected me) line where the ODO recorded more than actual mileage and a law suit occured. The result was the warrentee was extended by 5000 miles.