Illustrated Lecture on Tire Size, Ok, so I'm different! Options

[Porsche Rescue]

Members are often asking tire size questions and I usually respond by advising that they stay with something very close to the original equipment size. Unless the car is going to be frequently autocrossed or tracked I believe it is the best way to go. You don't change the gearing, create an inaccurate speedometer, reduce fuel economy or have to worry about clearance issues.
Today I replaced the tires on my 914-6 recently acquired from Eric Shea. He sent along a set of OE option 5.5x14" Fuchs ( beautifully polished by Eric). The car came to me with 6x15 Fuchs with 195/50x15 Dunlops.
Since the car will be a Sunday driver, cruiser (1000 miles to WCC '05 I hope), and since I bought the car for its originality, I wanted as close to stock tire size as possible. Sixes with the 14" Fuchs had 185x14 tires (same wheel/tire combo available on 911's as the "comfort"option). They are pretty much NLA. Their diameter was 25.6" with a cross section of 7.4". I bought a set of Michelin X 195/75X14's at Costco (used on Toyota Camrys and mini-vans!), diameter 25.4" and cross section 7.7". Close enough.
Here is a pic of the 195/50x15's beside the 195/75x14's. The 50 series, even though on a 1 inch taller wheel are nearly 3" shorter with a diameter of 22.7". No wonder gear ratios are changed (maybe a good thing for autocross) speedo is way off and fuel economy suffers.
Also, much less cushion and thus harsher ride (but more precise cornering) with the low profile.
All of the above applies to 4's as well. They came with 165x15 (as did the six with standard 15" steel wheels) which is exactly the same diameter as my 195/75's. The best modern size which is close is 195/65x15 (25" tall).