Just upgraded to 33s. How much is this throwing my speedometer off? In which direction? Programming suggestions?

 

How much difference depends on what you had.

I would get a ProCal to input the correct size.

An easy and accurate way to measure tire diameter is stick a little piece of chalk in the tread, roll over it twice, measure the distance between chalk marks, and divide by 3.1416.

 

Went from 255 to 285

 

Those numbers don't tell us the height.

But your speedo will read slow now by a few %, not much so I wouldn't worry with a programmer unless you have the spare cash.

 

You can check the difference at Tire size calculator so you'd be off by 5.4%. Hypertech and Daiblo make programmers that enable you to adjust for tire size changes.