'03 DII, stock wheels/tires and speedo 5mph slow. NAS speedo tolerances allow for +/- 10% typical GPS accuracy +/- 0.5 to 1.0 mph

 

you bring up a good point about the d1, i would assume that is because speedo is drivin off the t-box, so there really would be even more room for error with wrong tire size, everything being multiplied by the final drive ratio of the axles, where as with a d2 its measured via abs and individual wheel speeds, i dont see old componants causing this on the d1 really, its a spinning gear on a sensor, if its spinning then its reading, if not, then your speedo says 0, there really isnt a way for it to have any slip, other then catching and then not, which would cause a very erratic speedo

 

I experience EXACTLY the same issue with my 94 D1, as compared to the GPS system I use to find old bridges for my photography project. The Truck is actually going 5 mph slower than indicated, and this is with bone-stock wheels and tire sizes. The D1 does not use a speedometer cable, it is an electrical pick-up on the transfer case.

 

Now here is a monkey wrench thown into the whole thread...my '89 Camry with 196,000 miles on it and the speedo is dead on with my TomTom.
The '06 Sprinter (Mercede's)van that I drove for 18 months and the speedo was 5mph slow, from day one, OD dead on but speedo slow. The van had 100 miles on it when I got it, went through 3 sets of tires in 70,000 miles and the speedo was 5 slow no matter what.
My moms '05 F-250 Super Duty, dead on. My dad's '06 Civic, dead on, his '98 Corolla, 5 slow.
The '07 Volvo S40 that I rented in CA last year, 5 slow.
I think it all depends on the tide and what phase the moon is in.

 

WELL, my 84 Isuzu Trooper is spot-on with the computer GPS, but my 94 Disco 1 is consistently 5 mph slower than the speedometer indicates.