TeeJayHoward

I learned the hard way with my current car that there are two truths to winter tires:

1) Winter "Performance" tires are horrible in a proper snowfall compared to "Studless Ice/Snow" tires.
2) Skinny is better than wide... To a point, anyway.

I'm very strongly considering selling my current vehicle and purchasing a 997 for a year-round ride. After two years with Blizzak LM-32s, I'm never making the "Performance" tire mistake again. This means swapping in some proper snow tires when the white stuff falls. Unfortunately, my usual go-to for winter wheels and tires (TireRack) doesn't appear to have a tire setup for the 997 aside from winter performance tires... Even though they DO sell the X-Ice XI3 and Blizzak WS80, in the appropriate size (235/40R18) for the front wheels. I'm guessing they just don't sell the rear tire size. Heck, from what I've seen, nobody MAKES a studless ice and snow tire in Porsche's recommended 295/35R18. So, is it possible to buy four "front" wheels and tires, and mount them on the car? Will the car care that the rolling diameter on all four tires is identical (instead of the 25.5" / 26.3" factory split)? Or is this a stupid idea for some reason that's just not obvious to me right now?

And come to think of it, why doesn't the center diff care about the mismatched rolling diameter from the factory? I always thought that AWD cars flat out required it. Is it just a different final drive on the front diff?

Scooby921

The best winter tires I've ever used were in the "performance" category and got me through several feet of snow. Even got me out of a 4ft deep snow drift. Don't write off all "performance" winter tires because you happened to discover that "Blizzak" on the sidewall doesn't actually mean much. Bridgestone has a few Blizzak models which are fantastic winter tires. They have just as many that suck. I don't know if they'll be any good, but it looks like Pirelli has a winter tire in the correct sizes (Winter Sottozero Serie II). If you're going to mount these on a separate set of wheels you could also look at different diameters. Most people go smaller in diameter for a winter wheel and tire, but Michelin sells the Pilot Alpin in wider sizes for 19" and 20" wheels to fit most of the current crop of performance cars. I'd buy a Pilot Alpin over a Blizzak every day of the week.

Skinny is better than wide for cutting through slush and avoiding hydro planing on roads that have snow melt. On a homogeneous snow / ice surface the wider contact patch is more grip for stopping, going, and cornering.

It's likely that the final drive ratio for the secondary drive axle accounts for the rolling diameter difference. It's pretty standard for the gear ratios to be slightly off (less than 1%) just to keep the gear teeth loaded and avoid NVH issues from gear rattle. Most of the AWD systems my employer sells have front and rear ring gear ratios around 0.3% different. The the manufacturer knows they are going to have a 1% or 2% speed offset on tire diameters it's pretty easy to build that into the gears.

Mike Murphy

TeeJayHoward

I'd love to give them a shot, but they don't make them in 997 sizes (295/30R19 and 235/35R19).