Since the newer tibs come with 17's from the factory Im trying to figure out if I should get new 17's with stock sized tires or go up +1 and get 18's. Im worried about weight, rolling resistance, and braking. Will a +1 in size make a big difference or should I stay with 17's?

 

Changing the wheels by 1" won't let you see any difference really. You may also consider a slightly smaller tire size also.

http://www.miata.net/garage/tirecalc.html Use this to see changes in your tire.

 

You can get a light weight 18 that will weigh less than your stock 17's. Depends on how much you're looking to spend.

 

It really comes down to personal taste. 18's won't throw things off much since you can still get a tire that's the right height. You'll have less sidewall so your ride will be rougher.

I think 18's look too big on a Tib personally so I went with 17's. You could look through the HA garage and probably find a decent selection of wheel sizes to compare how they look.