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thstone · 05-30-2019, 09:16 PM

Its complicated.

The key is the load index. Two different tire size combinations can have the same (or very close to the same) load index and therefore, will have similar tire pressures. That is what Porsche did for the 17 and 18 inch tires on the Boxster. If the load index changes between the old and new tires, then the tire pressure is likely to change. This is the general concept, but of course, its actually more complicated than that.

This paper from Toyo tries to explain it and gives an example for a +1 fitment using a different type of tire and different sizes. In this case, the load index changes and so the tire pressures change. Using the example as a guide, this should help you to determine the best pressures for your exact situation:

https://toyotires-1524598101.netdna-ssl.com/media/1541/application_of_load_inflation_tables_20151020.pdf

05-30-2019, 09:16 PM	#3
thstone Certified Boxster Addict Join Date: Nov 2010 Location: Los Angeles Posts: 7,669	Its complicated. The key is the load index. Two different tire size combinations can have the same (or very close to the same) load index and therefore, will have similar tire pressures. That is what Porsche did for the 17 and 18 inch tires on the Boxster. If the load index changes between the old and new tires, then the tire pressure is likely to change. This is the general concept, but of course, its actually more complicated than that. This paper from Toyo tries to explain it and gives an example for a +1 fitment using a different type of tire and different sizes. In this case, the load index changes and so the tire pressures change. Using the example as a guide, this should help you to determine the best pressures for your exact situation: https://toyotires-1524598101.netdna-ssl.com/media/1541/application_of_load_inflation_tables_20151020.pdf __________________ 1999 996 C2 - sold - bought back - sold for more 1997 Spec Boxster BSR #254 1979 911 SC POC Licensed DE/TT Instructor Last edited by thstone; 05-30-2019 at 09:22 PM.