So, I got some 16x8s off the ol' Craigslist today for $200 with some used, but still usable LT265/75R16s mounted.
So, I did the only thing I could think of, and borrowed the TomTom to check out how far off the speedo is, because it should be about 10% off I guessed.

When I started out down the alley, I noticed that the speedo was fast; needle on 15, GPS says 12. Chalk one up for discrepency?
Then cruising down the residential road out to the main street, the speedo was spot on, 25 = 25.
When I get out to the main road (45 zone), the speedo is slow; needle on 45, GPS at 49.

Am I doing it wrong? Am I crazy? Is something broke?
Help! Help! I'm being repressed! (Sorry, I have Monty Python moments.)

Any pointers are greatly appreciated.

 

What is your name?
What is your quest?
What is your favorite color?

Anyhow, GPS is accurate to about +/- 20 feet. At slow speeds where the distance traveled between GPS satellite samples is less than the margin of error, you end up with an average of where it thinks you might be. I've sat at stop lights for several minutes and seen my GPS tell me I've moved 1 MPH to the right, 2 MPH to the left, sat still for a moment, 1 MPH to the rear and then didn't move for 10 seconds then 1 MPH to the rear as I was driving forwards. At higher speeds, the differences in your location when the GPS readings are made are greater so the accuracy increases since the reading are +/- 20 feet from where the reading is made.

The faster you move the more accurate it is, when you move more than 20 feet (the average accuracy of GPS) per reading.