Rear Tire Alternatives... (sorry, this is a re-post)

[porsche996tt_tpe]

Suppose the front tires spec remain the same, the rear set changed to 285/35/18. Will this ruin the diff? Or any other close tire alternative with 18" rear wheels?

Is it possible to disengage front diff and drive normally?

Thanks!

[White987S]

Not sure. Maybe consider a 295/30/18. I don't know what size you are running up front.

On my 993 Turbo (also AWD, like you) I run 295/30/18 in the rear with 235/40/18 in the front. No issues. Stock OEM sizes were 285/30/18 and 225/40/18.

I know you can convert the 993 to rear wheel drive. Not sure about the 996. I suspect you can, remember the 996 GT2 is rear wheel drive. I don't recommed you convert away from AWD.

I also DON'T recommend that you mix tire brand or tire models. So, if you are replacing your rear tires, make sure your fronts are a matched set.

[Dus10R]

I cant remember what the spec is, but it has nothing to do with tire width, but the difference in diameter front to back, which will change as a result of changing widths. I think it says in the owners manual what difference is allowed. i used to go to Tire Racks website and look up the specs (diameters) on the Porsche approved "N" rated tires for the Turbo and then calc. The diameter differences. You want to keep them as close as possible because the PSM and traction control were designed with very specific parameters based on their (Porsche's) tires. So not only does running different diameter tires effect the diff. It also effects the electronic sytems. I switched over to 19's once on my Turbo and it didn't take me long to realize the car was not deigned around thier use and the different sized tires they required. The PASM system would interfere way earlier than it did with the stock sized wheels and tires when pushing the car hard. I would have to turn off the PASM if I was going to drive hard because if I didnt it would try applying a brake during a hard corner and it would really unnerve me and screw up the balance of the car. Went back to stock wheel and tire sizes quickly after that and all was well. The 964 Carrera 4 cars were even worse. They have a completely different AWD system that has more in common with Audi's Quattro than all other AWD Porsche's except the 959. Highly complex and very very finicky to wheel and tire size. Great snow car.

If you dont need the AWD, you can remove it. It drops 100+ pounds off the car and cures the common understeer, but its still no GT2. Totally different suspension components and hubs.

[King James]

Check the Miata tire size calculator, lets you do direct comparisons. http://www.miata.net...e/tirecalc.html

Its saying the 285/35 is 3.4% smaller so not the same ratio as 295/30. Will it cause problems? Well the rear tires will be smaller than the front if your running the 235/40 so there will be a variance between front and rear. Can the diffs soak up 3.4% change in ratio? Well, many guys go to the 315/25 and it's 3.2% larger and seems to be fine. So i'm guessing yes.

Why not buy the proper size tire or are you getting a deal?

[Aerodude]

Since I'd like to try out the Michelin Pilot Super Sports in 18", I've looked into this as well. For the rears, if I were to go to a 285/35 (which is 25.9" diameter), I'd go with a 225/45 in the front (same diameter). Disengaging the front diff seems like a drastic measure (unless you want to do it for other reasons) just to go for a different size tire from OEM in the rear. Also, note that the larger diameter could be an issue if your car is lowered a lot (not an issue for me).

Let us know what you decide to do -- I might follow your lead :).

[porsche996tt_tpe]

The 285/35/18 has much cheaper alternatives (Taiwanese and Korean tires) than 295/30/18's, which is why I'm asking if this is OK. The rear tires just wear off too fast (even for regular driving, they go out at about 10~15K KM) and cost too much...

So, if I change the front to 225/45, the rear will work with 285/35?