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Originally Posted by Mrbig
Question is how much are you saving using a 17inch setup with rotational mass over a 18inch wheel for speed purposes???

We had a customer that actually did a comparison on another forum, and this is what he found about the weights:
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So I made a spreadsheet of tire sizes vs ARC8 wheel sizes and weights to see if the 17" is worthwhile. I found that while some 17" tires weigh less than their 18" counterparts, others weigh a bit more.
Examples:
Michelin PSS is 1 lb heavier in 245/40/18 vs the slightly lighter 245/45/17.
Continental Extreme DW is 1 lb lighter in 245/40/18 vs the slightly lighter 245/45/17.
Continental Extreme DW is 1 lb heavier in 255/40/18 vs the slightly lighter 255/45/17.
Sumitomo HTR Z III is 1 lb lighter in 245/40/18 vs the heavier 245/45/17.
So the weight of the tires seems to be a wash, which means that the 17" is definitely lighter in almost all cases because the wheels are 1.52 lbs lighter in 17". But it's not a big difference, and if you buy the wrong tire, you could end up with 17" setup that is actually heaver than an equivalent 18" setup.
For the record, the lightest tires were PSS and Conti Extreme DW. These both weigh 23 lbs in 245/45/17. The Contis (but not PSS) are also available in 255/45/17 and at 275/40/17, both weighing in at 25 lbs.

Hopefully that helps. With less unsprung weight on the corners, the rotational mass should also be reduced approximately the same given everything else is identical between the 17" and 18" setups.