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Discussion Starter · #1 ·
I work on VWs as a hobby and just finished writing up this calculator. It's web based so you don't have to download an .exe file. It's not Porsche specific but you can use it for any car. In addition to the standard wheel/tire diameter/width difference it also tells you changes in offset, the wheel/tire moving inboard and outboard, has tables for your gear and final drive ratio (if you want to calculate 6th gear engine rpm), and explains some output information in detail to help you interpret the data.

I'm not a professional programmer but any suggestions are welcome...within my limits! Try it out and feel free to spread it if you like it!

Tire and wheel calculator with engine rpm and offset calculations
 

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Seems like there may be a glitch with the spacer calculation. For example if i use parameters of 295/35 19x11 for "current" and "planned 1" columns, but add a value of 15 for spacers in "planned 1", the results for "Wheel/tire outboard movement (positive is sticking out, negative is tucked in. inches):" show a negative value of -0.591 inches. I'd imagine that with the same exact wheel dimensions, that adding a spacer would always give a positive value for outboard movement?

:cheers:
 

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Discussion Starter · #5 ·
Thanks for pointing that out, it's now fixed. I added spacer to offset when I had to subtract. Glad you spotted it!

I also added a theoretical top speed (redline limited) if you input your tire/wheel size, top gear, and final drive.
 

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Somebody might want to double check me to confirm these numbers, but after a little research I found the following data:

987.1 6-speed - 6th gear 0.82, Final drive ratio 3.88
987.1 5-speed - 5th gear 0.84, Final drive ratio 3.75
987.1 Tiptronic - 5th gear 0.74, Final drive ratio 4.16

987.2 6-speed - 6th gear 0.88, Final drive ratio 3.88
987.2 PDK - 7th gear 0.62, Final drive ratio 3.88
 
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