I'll be lowering my car next week and since I've never been a fan of the Track Pack wheels I also decided to swap my wheels and tires. I"ll be putting on staggered Mickey Thomson SC-5 wheels 18X9 in front and 18X10.5 out back.

I've read a ton about tires and wheels but I'm still confused about wheel width and measured tire width when downsizing. I'm considering 255/45 18 for the front and either 275/40 18 or 285/40 18 (depending on the tire brand I finally get) for the back.

The 285s are .02" smaller than the original wheel diameter (27") measured on a 10" rim. The 275s are .3" smaller measured on a 9.5" rim.

Will putting these on a 10.5" rim have an adverse effect on overall diameter? (I'm thinking yes, but just want to make sure)

What is a an acceptable (safe) variance in overall diameter?

 

Yes, the overall tire height and thus rolling diameter will be affected. I wouldn't say there is a uniform limit to the variance in diameter, but it will affect the accuracy of your speedo, as well as the max speed you can achieve in each gear. If that isn't a major consideration for you ~0.5" difference or so should be ok.

Take a look at this calculator for details in speedo error and overall size.
http://www.discounttire.com/dtcs/infoTireMath.do

EDIT: One other note. Shorter tires on the back end will also have a steepening effect on your final drive. So if you have 3.55 with 27" tire when you drop to 26.5" tire that would be equivalent to having original tires with 3.62 gears

 

5.M0NSTER thanks for the info. I think I'll stick with the 285s then.