Main Menu

1989 964 wheels and tires

Wheels and Tires

Vehicle Information: Model: 964; Year: 1989; Body Type: Coupe; Total Mileage: 147,398;

Porsche 964 Wheel Tire Questions;I own a 1989 (K) Porsche Carrera 4 (964). When I purchased the car it had Design 90 wheels 6X16 front and 7X16 rear. After reading my owners manual and my copy of the book Enthusiasts Companion I discovered that the wheel size should be 6X16 front and 8X16 rear. I was lucky enough to purchase a set of 4 each Design 90 wheels in the size of 8X16 at a reasonable price. The correct size Design 90 wheels of 6X16 front and 8X16 rear are now installed with tires sized 205/55X16 front and 225/50X16 rear. I like the look of the stock wheels, but they do not fill out the wheel arches very well. I purchased a set of 4mm non-hub centric spacers and installed them on all 4 corners to see if it would look any better. In my opinion this did slightly improve the looks, with negligible changes in performance. Currently I need to replace my worn out tires. I am looking for your opinion on my proposed wheel and tire set up. I have read every post on the PCA site and think I am ok with my plan. I am just looking to validate my understanding of the information. I want to take the Design 90 wheels I have left over and put the 7X16 wheels on the front with a 225/50X16 tire and put the 8X16 wheels on the rear with a 245/45X16 tire. My goal is to improve the look without hurting or molesting the car and perhaps reduce some of the under-steer. The car is used primarily for sunny day drives and 1 Auto-cross per year.My understanding of the AWD, PDAS and Anti Locking Brakes is that wheel diameter is critical. Please review my calculations to see if I am on the correct path. Your suggestions / corrections are highly appreciated! Stock wheels Design 90, 6X16 and tires 205/55X16 front pair205mm X 55% = 112.75mm one side-wall height112.75 X 2 = 225.50mm total top and bottom of tire side-wall height16 inch wheel = 406.40mm in height406.40 + 225.50 = 631.90mm total stock front tire wheel height/diameter in mm631.90mm = 24.87795 inchesStock wheels Design 90, 8X16 and tires 225/50X16 rear pair225mm X 50% = 112.50mm one side wall height112.50 X 2 = 225.00mm total top and bottom of tire side-wall height16 inch wheel = 406.40mm in height406.40 + 225.00 = 631.40mm total stock rear wheel height/diameter in mm631.40mm = 24.85827 inches 24.87795 inches front-24.85827 inches rear =.01968 inch diameter difference from front to back tires------------------------------ MY PROPOSED TIRE SIZES FRONT AND REAR------------------------------ Proposed front wheels Design 90, 7X16 and proposed front tires 225/50X16 225mm X 50% = 112.50mm one side wall height112.50 X 2 = 225.00mm total top and bottom of tire side-wall height16 inch wheel = 406.40mm in height406.40 + 225.00 = 631.40mm total proposed front tire wheel height/diameter in mm631.40mm = 24.85827 inchesProposed rear wheels Design 90, 8X16 and proposed rear tires 245/45X16245mm X 45% = 110.25mm one side wall height110.25 X 2 = 220.50mm total top and bottom of tire side-wall height16 inch wheel = 406.40mm in height406.40 + 220.50 = 626.90mm total proposed rear tire wheel height/diameter in mm626.90mm = 24.68110 inches 24.85827 inches proposed front tire-24.68110 inches proposed rear tire =.17717 inch diameter difference from proposed front to back tires-------------I BELIEVE THE REFERENCE DIAMETER FOR EACH TIRE IS 25-INCHES------------- So if the stock front tire 205/55X16 is 24.87795 inches in diameter then25inches/100% X 24.87795inches/X%25X = 2487.795X = 99.5118%100% - 99.5118% = .4882% from 25 inch reference for the stock front tiresSo if the stock rear tire 225/50X16 is 24.85827 inches in diameter then 25inches/100% X 24.85827inches/X%25X = 2485.827X = 99.43308%100% - 99.43308% = .56692% from 25 inch reference for the stock rear tiresThen the difference in percentage of diameter between the stock front and stock rear tires is99.5118% front - 99.43308% rear = .07872% difference between stock front and stock rear tires ---------------PROPOSED FRONT AND REAR TIRE PERCENTAGE CALCULATIONS--------------- So if the proposed front tire 225/50X16 is 24.85827 inches in diameter then 25inches/100% X 24.85827inches/X%25X = 2485.827X = 99.43308%100% - 99.43308% = .56692% from 25 inch reference for proposed front tireSo if the proposed rear tire 245/45X16 is 24.68110 inches in diameter then25inches/100% X 24.68110inches/X%25X = 2468.11X = 98.7244%100% - 98.7244% = 1.2756% from 25 inch reference for proposed rear tireThen the difference in percentage of diameter between the proposed front and rear tires is99.43308% front 98.7244% rear = .70868% difference between proposed front and rear tiresMy summary on the stock tire sizes is Front tire 24.87795 inches diameter at 99.5118% out of the 100% 25-inch reference diameter Rear tire 24.85827 inches diameter at 99.43308% out of the 100% 25-inch reference diameterDifference between stock front and stock rear tires is .01968 inch diameterDifference between stock front and stock rear tires is .07872% My summary on the proposed tires sizes isProposed front tire 24.85827 inches diameter at 99.43308% out of the 100% 25-inch reference diameterProposed rear tire 24.68110 inches diameter at 98.7244% out of the 100% 25-inch reference diameterDifference between proposed front and proposed rear tires is .17717 inch diameterDifference between proposed front and proposed rear tires .70868% My first conclusion is that my proposed installation of 225/50X16 front tires mounted on 7X16 Design 90 rims and 245/45X16 rear tires mounted on 8X16 Design 90 rims will work. I believe that each tire is well within the 6% reference of the 25-inch diameter requirement as well as the 4% reference difference allowed from front to rear tire diameters.Please check out the second way I calculated my numbers. Does this make sense, or am I wrong?100% = 25 inches100%/25 inches X X%/1 inch25X = 100X = 4Stock front tire diameter of 24.87795 inches X 4 = 99.5118% or 100% - 99.5118% = .4882% smaller than the 25-inch referenceStock rear tire diameter of 24.85827 inches X 4 = 99.43308% or 100% - 99.43308% = .56692% smaller than the 25-inch reference The difference between the stock front and stock rear tires is 24.87795 24.85827 = .01968 inches in diameter, so .01968 X 4 = .07872% difference between the stock front and stock rear tires ----------------------------------------------------------------------------------------------------Proposed front tire diameter of 24.85827 inches X 4 = 99.43308% or 100% - 99.43308% = .56692% smaller than the 25-inch referenceProposed rear tire diameter of 24.68110 inches X 4 = 98.7244%or 100% - 98.7244% = 1.2756% smaller than the 25-inch referenceThe differen

You must log in to view answers associated with technical questions. Join PCA to participate in our community.
No votes yet