Quote:
Originally Posted by PaulE
In stock sizes for the Boxster, the 17 and 18 tires have practically identical outer diameters. My 18 inch wheels have Michelin Pilot Sport PS2 N-3 tires, and the 17 inch wheels have Toyo Proxes T1 Sport tires. Interestingly, to me anyway, was that there was no real weight savings with the 17 inch wheels and tires compared to the 18s. 1 front 17 was 41.4 lbs vs 1 front 18 at 41.6 lbs. At the rear, 1 17 was 46.8 lbs and 1 18 was 48 lbs. The 17 inch tires were brand new, the 18s were not - 18,500 miles on the front tires and 5,700 miles on the rears, but still have lots of tread left on all of them.
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Welcome to the forum, and thanks for the real world data. Even though not rigorously scientific, this data does indicate there is not much if any advantage in weight of one over the other after including the tire weight.
Also as you noted, there is usually no significant advantage in tire diameter with a 17" vs. 18" due to the street tire industry's dedication to that +1/-1 tire sizing concept, since they don't want anyone's speedometer or traction control to be fooled.
It may seem obvious, but the wheel is there for one reason - to support the tire. So it comes down to first selecting the tire size that you can and want to run, and then selecting the wheel to match. If you want to run something wider than a 255 tire, your choices for fitment on a 17" wheel are very limited, and will probably not get any better in the future. If you plan to stay with 255 and smaller tires, the 17" wheel is a good lower cost option.
One caution with 17" wheels: If you ever shim your rear toe links to adjust bump steer, it can cause the retaining nut where the rear toe link bolts to the upright (wheel bearing carrier) to make contact with the inside of the wheel. This happened with my OEM 17" wheels that I use for rain tires, and I didn't realize it until I was bolting them on in a hurry for a wet track session. I was able to file the nut down about 2-3 mm to clear this clearance issue, but this took off the nylon part of the lock nut. I later drilled and safety wired. This is probably not an issue unless you are seeking the last fraction of an improvement in suspension set-up.