The 18" wheels they used weighed less than the factory 17" wheels

Quote "We needed 18-in. wheels, and worked with Eddie Lee at Mackin Industries to find a set that would fit the FR-S without any modification. He set us up with a set of Advan RG II wheels, measuring 18 x 71⁄2 with a +48 mm offset. Although expensive, the 18-in. diameter Advans actually weigh less than the stock rims (an RG II weighs 17.2 lb.; the factory wheel weighs 20.4 lb.). Plus the Advans fit perfectly with no rubbing or clearance issues." End quote

They used Star Spec Dunlops for both the 17" and 18" tires

215/45R17 is 24.7" diameter weight of tire 22# wheel weight 20.4# total weight 42.4#

235/40R18 is 25.4" diameter weight of tire 25# wheel weight 17.2# total weight 42.2#

So the tire wheel weight is withing .2" of a pound, diameter increased by 0.7" and the times fell off by 6 tenths of second and track speed was down by 1.5mph with the 235/40R18 Dunlop vs the 215/45R17 Dunlop