I would probably wait for them to wear out too.

 

Not sure how different the z's speedo readings are, but for my g35 coupe, 275/295 it seems like the fronts would be +1.89% and rears +1.55%, thus causing a pretty significant change in speedo readings. Do the front and rear differences simply add to the total difference in speedometer reading? I'm trying to see if I can do a bigger setup on my next set of tires vs my 255/35/19 front and 285/35/19 rear setup now, but I would like to keep my speedo pretty accurate

 

Your speedometer reading is only affected by the overall diameter/circumference of the wheel and tire size, not the width. If you go up from 17/18/19 inch wheel, you need to drop the size of your tire sizewall accordingly. This will maintain an accurate speedometer reading. Also don't forget that as you lose tire tread the speedometer reading will varry as well. You'll be going slightly faster on new tires and slightly slower on old tires +/- about 1-2 mph.

 

QFT ^

 

The speedo only reads off the rear tire. Staying as close to stock outer diameter (OD) is ideal , but not always possible do to tire selection.

 

Ok thanks for the info, I figured it makes sense that the outer diameter would change even just changing widths due to how stretched or how much more the tires buldge. But I guess overall the diameter change is relatively small between my old 255/35 rear and new 285/35 rear, so it makes sense that the speedo probably won't change much.

Thanks terrasmak, I definitely appreciate your insights, you even convinced me to just pickup a setup of kw v2 coils to replace my fortune auto coils. I have a set of 275/35's sitting around so when my 255 front's are done I'll try putting those up front and then maybe size up to 295/35 rears or something similar once my rears are done as well.