Speedometer Changes with tire sizes

Not sure if I should put this on the wheels and tires board or here, but here goes. How far off should I expect my speedometer to be if I change the factory 265/65R17s to a set of 285/70/17s? And what's the easiest/cheapest way to get your speedometer calibrated?

 

mine is only off by about 3mph at 65. not a big problem for me, just add 3

 

Here ya go :

http://www.tacomaworld.com/forum/tirecalc.php

Dont count on getting the speedmeter re-set for diff tire sizes.
Toyota computers are not set up for it, apparently.

This has been pretty much beaten to death, but keeps coming back; try a search.

 

I also went from the stock 265/65R17's to 265/70R17's, and now my speedo reads exactly what my actual speed is. Before I upsized, my speedo would read 3 mph over my actual speed. I know this because when I comute to work, there are a few stationary radar signs that I pass and they flash your speed as you pass by.

 

you can correct the speedo for tire sizes. it requires the use of a dakota digital speedo box that has to be spliced into the wiring harness. it allows you to change the setting if you ever change tire sizes and corrects the speedo accordingly. i plan to get one for mine soon as mine is about 15 mph off now.