I've got a set of 275/35ZR20s coming for all four. They're .4" larger than stock.

Do I need a new tune? Or is this something I can adjust myself in the tuner?

 

As long as you stay within 3% of the overall tire diameter of the factory tires, you should be OK. If you go outside the 3% rule, most manufacturers say your ABS and traction control could be compromised.

 

I have the same tires on my ride and it didn't effect the Speedo at all. I double checked it with my GPS

 

Post pics of your new set-up when it's done.

As far as a tune, you would only need it if you're concerned about your speedo being 1.5% low. When you're going 70, it will say you're going 71. But then again, if you drive 10,000 miles, your odometer will say you went 9,850. Another effect it will have is on your final drive ratio: your 3.55 will be a 3.49. Goodbye acceleration! Just kidding...

 

I have 275/35/zr20s in the rear and it didn't affect my speedometer.

 

Also, it is something you can adjust with your tuner.

Oh, and Nitto has a 305/30/20 now for those interested! Yeah!

 

You'll need to compare revolutions/mile between the old and new tires, then make the adjustment in the tuner (at least that's all I had to do in the Predator).

 

you can adjust it yourself in the sct xcalibrator. i saw the option on the tuner, but i didnt go into the menu. something about measuring the distance for the revolution of the tire.