´71 911T - 14" or 15" Cinturato CN36s?

[RetroGuy]

My ´71T came factory equipped with 14" x 5.5" Fuchs "comfort" wheels, which I am currently running, being shod with 185 x 14" Vredestein Sprint Classics.

As I also have a set of 15" x 6" Fuchs deep sixes ready for installation, I asked myself the following:

Since it´s time to change out the 10 yr. old Vredesteins (which always felt a bit too soft for my liking - I think due to the rather high side walls (measuring out to approx. 148 mm) I was wondering whether I would notice a difference between the Pirelli Cinturato CN36 in their 14" size (wheel diameter 614 mm, equalling a side wall measurement of about 130 mm) and these tires in their 15" size (wheel diameter 637 mm, equalling a side wall measurement of about 129 mm)? I am well aware that between my 14" Fuchs rims and the Deep Sixes there´s also a difference in width of 0.5" which could make a difference?

The 14" combo would save about 1.3 kg/2,85 lbs per wheel. Also the car would sit slightly lower.

I am well aware that aesthetic-wise the 15"-version is preferable, but at this point I´d primarily be interested in the performance aspect of this ;-)

Many thanks for your expert advice!

Stefan

[davep]

The tall sidewall of the old 14" tires was part of the comfort aspect of the package; they were intended to be soft to help provide a more comfortable ride. So not a performance option in that respect. A lower profile tire would certainly stiffen things up and can assist handling. The 14" rim adds a bit of performance from a gearing point of view and also from the reduced mass. The spinning wheel acts as a flywheel and a smaller mass spins up more readily, and requires less braking. So, a lot of wins there depending on how you drive.
The wider 6" rims do offer some advantages also, but as you go up in size and width to improve grip, you fuel consumption also goes up. I remember some 40 years ago with my 914 1.7 when I went from the 165x15 to the 185/70-15 and 5.5" to 7" rims the fuel mileage went from about 45 MPG to about 30 MPG. It certainly handled better in corners so the trade-off was worth it at the time. Two sets of rims does give you options for different purposes.

[Arne]

I doubt that you'd notice much handling difference between either of the CN36 choices. The two differences (unsprung weight and wheel width) are insignificant in normal road driving.

Regarding the Vredsteins, a local guy (and successful Miata racer) has 185/70-15 Vreds on his '77 911S and he hates them. His comment to me was that "the sidewalls are way too soft." So again, I think either CN36 size will be an improvement for you.

But for myself, the key issue would be the lower overall gearing of the 14" CN36. The gearing (and speedometer/odometer accuracy) will be noticeably shorter. First gear will be even less useful than it is now, and you will be turning higher revs to keep up in traffic. With the inaccurate odometer, it is harder to measure, but fuel economy will suffer as well.

So for me, I'd go with the 15" choice for everyday uses.

[Arne]

Another option you may want to consider would be Michelin MXV in your original 185HR14 size. Not quite as vintage looking as the CN36, but I'm going to guess they'd handle better than the Vreds do, and they are possibly the only other performance rated tire in your original size.

[runsub5]

Another option you may want to consider would be Michelin MXV in your original 185HR14 size. Not quite as vintage looking as the CN36, but I'm going to guess they'd handle better than the Vreds do, and they are possibly the only other performance rated tire in your original size.

I recently was asking a similar question of Longstone Tyre concerning proper size and brand for 14" wheels. They agree with you. Their recommendation was to go with the Michelin MXV-P in 185 HR 14. This would maintain originality and and keep proper speedometer/odometer readings.