max allowed tire size error ?

[acadian_dad]

I assume that since PSM, ABS, etc. look at relative rate of rotation of front and back wheels there must be a tolerance in the matching of front to rear tire circumferences.

Eg. my Boxster-S has the OEM 235/40-18 on front and 265/40-18 on the back - these work out to 2027 mm and 2102 mm circumference respectively. I've seen various recommendations for R-cmpd alternatives (245/40-18

& 275/40-18) seem to be a common setup - both end up being about 1.2% larger - but Mich PSC's are not available in the 275/40 so still looking at other options.

How much mismatch from the OEM sizes are allowed before PSM, ABS, etc. are messed up ? I suppose in theory its the mismatch of the ratio of front to back - not the absolute sizes - that will mess things up - but not sure.

[berty987]

The car is balanced with the tyre options it has from the factory , fitting larger tyres will cost more and give little benefit. Its generally recognised that slick tyres on a 987 can lead to issues with oil surge on the circuit , so again its probably going to give more problems than benefits. I dont think the front to rear ratios are as much of an issue to the ABS/ ESP as the differences across the same axle. The rolling radius is different between a 2.7 and 3.2 and 3.2 with 19" wheels. There are many cases of people fitting 19" wheels to a 2.7 without issue, so I dont think you will have many if any problems.

[acadian_dad]

Thanks for the thoughts. I'd love to keep the factory sizes, but they are not available on the R-comps and I just want to ensure that if I go this route, I won't have any ABS / PSM surprises. The issue you mentioned about oil starvation is not something I had heard about on this car. Can you provide any links to more info on that ?

Edited October 25, 2008 by acadian_dad