....

 

If you are still AWD the tire circumference is too different, you will pop your diff.
You need to go 235/40/18 on front to run AWD with 315/30/18 on back I believe.
It does not matter if you are RWD.

 

I have seen this kind of thread a few times now, so I thought I would contribute by posting this tire calculator that I use to determine what size tires would be appropriate. It is an Excel spreadsheet where you enter width, percent and wheel size and it will return tire diameter, width in inches, circumference and the difference from the base size.

 

I have not tried it myself, but I would not advise it.

 

Tire calc spreadsheet is very handy but you really need to check out the specs for each individual brand of tire...despite the same "size", the dimensions can be very different (e.g. a 225/40/18 can be the same rolling diameter as a different brands 245/40/18). Re OP, you should be OK if that is the actual spec difference between the front and back.

 

not to hijack this thread but if we make the question more general...
how much of a difference is considered ok? (from stock, between front and rear)
And how would the answer change in case of a GT2/3 (RWD only and no PSM).
I've seen guys on Rennlist run 335's on GT3 and I guess everything works fine for them?

 

using tiretrack info on actual tire sizes:

Stock setup Contactsport2 225 40 18 and 295 30 18 rear has difference is 0.37 inches.

if u mixed (not ideal) Conti 225 40 18 and Michelin 315 30 18 difference is 1.2 inches
but Conti's 235 40 18 and 315 30 18 is 0.12 inches. (GT2 setup)

if all PS2 tyres, then interestingly 225 40 and 315 30 rear diff is only 0.44 - this is because michelin 225/40 diameter is bigger than the same conti 225 40. (25.3 versus 25.09 inches)

The Viscous LSD works by hydrodynamic friction of the viscous (thick) silicon oil coupling two sets of plates. That oil loses effect over the years / if over heated.
So if you circumferential difference is say 1.2 inches - i.e with conti 225/40 and michelin 315/30 (I am thinking about it), it is possible you MAY overheat the LSD and it will lose effect. Some manufactueres deliberately have a bit of turn in the LSD so that it acts faster.

Bottom line, if you have bold rears and new fronts, you WILL have 1.2 inch difference, so...it is likley OK.

Disclaimer : be very careful messing around, as the tyres are what keep you alive.

 

if you have 0.5 cm of tread wear on the stock rear and none on front tyres, the circumferential difference is 1.25 inches.... that will very likely have been factored in the design for the viscous LSD, so, it is very likely that 225 40 Contis and Michelin 315 30 PS2 combo, will NOT affect the LSD. just my opinion.

 

I would say good move, even for just keeping compounds and tire age the same front and rear.

 

done 300 miles with new pilot sport 315/30/18 and left the stock conti 225/40/18 on front. cannot tell of any negative handling traits or viscous diff overheating. seems within tolerance.