chuckla

I am thinking of using my MM replica wheels with snows (from my '03 C4S) on my '06 C4S. Dealer says spacers (used 20mm spavers on '03) not necessary on the '06 and that handling should be OK but not as good as with 19" wheels. Of course, should give a little better snow traction with the smaller wheels. Anyone think this is a good or bad idea?

purrybonker

I think it's a great idea. 18's are probably an even better real-world handling alternative than the 19's anyway, truth-be-known.

I vaguely remember reading one of those tech "shoot-outs" where they put increasing diameter wheels on the same car and did lap times. Can't remember the details, but I seem to recall that 17 inch wheels gave the best overall results.

Big wheels are a bit of a marketing/design fad that's gone a bit nuts.

Plus, look at the much better tire selections/prices in 18 inch rubber!

Edgy01

The only wheel that will take tire cables (for snow) are the 18s. Only ones that give you the necessary clearance.

David A

How about 996 Turbo wheels?

rockrich

Won't 18 inch wheels lower the ground clearance? How much will this be noticeable?

Edgy01

Look at the aspect ratio of the 18 inch tire for that answer. Overall diameters (with rubber) should be very similar,-otherwise you would have to adjust your speedometer as well!

chuckla

Dan-Service mgr said speedo would be slightly off. Think he said I would be going slower than speedo says. My take is that it seemed from what he was saying that the difference wasn't important. And smaller wheel might affect ground clearance in negligible way.

Nugget

That doesn't really make sense. Comparing front stock sizes...

The 235/35ZR19 tire that's stock on an '06 C4S is a 25.5" diameter wheel (tire and rim) with a circumference of 80.0" at full tread.

Moving to a 235/40ZR18 tire that's stock on an '06 C4 is a 25.4" diameter wheel (tire and rim) with a circumference of 79.8" at full tread.

That's just a tenth of an inch difference in diameter, which means you can expect the 18" wheel and tire to lower the car by 1/20th of an inch and for there to be 1/20th of an inch more wheel well clearance.

In the rear you get a bit more of a difference -- a 26.2" diameter drops to a 26.0" diameter, which buys you 0.10" of extra wheel well clearance moving to the 18" wheel.

I have a tough time visualizing how such a miniscule difference could matter when it comes to snow chains or traction cables. What am I missing?

Incidentally, this also means that moving from 19" to 18" means your speedometer will read 0.3% too fast. In other words, the speedometer will read 60mph when you're really only going 59.8mph. Hardly seems worth worrying about, you get about the same amount of inaccuracy comparing new tires to treadworn tires and it's well within the margin of error for a speedometer anyway.

Edit: Looks like the 2003 C4S had smaller diameter rears than the 997s do. Numbers for the front tires are the same as above, but for the rears you end up going from a 305/30ZR19 to a 295/30ZR18 (997 has a 295/35ZR18 in the rear). This would yield a more meaningful difference, you'd end up going from a 26.2" diameter to a 25.0" diameter. That's a 4.7% difference in rolling circumference, so your speedo will read 60mph when you're really only going 57.2mph. Of course, if you're buying new tires you'd probably just mount 295/35s on the '03 rims and keep with the original numbers above.

chuckla

Thanks Nugget. Since I never got past 4th grade math, you are my hero.