Kerry's 330 GT Restoration

Tires

One area that I needed to address were the tires that were on the car.  They were Pirelli CN 36 in 215VR60-15 size.  The spare was a bald 225VR70-15.

PirelliCN36s.JPG (208247 bytes)

One problem that I always had was that the 60 series tires dropped the ride height by almost an inch and one-half.  That was enough that one ended up scraping on driveway entrances, speed bumps, etc.  The original tires were 205-15.  These were the original type of radial tires which had an aspect ratio of .80 or would be a 80 series tire in today's nomenclature.

The aspect ratio is the sidewall height divided by the tread width.  The lower the number, the wider the tire is relative to the sidewall height.

The problem is that the 205/80-15 size tire is no longer available.  Coker Tires does make old sizes of tires, but not in that particular size.  They are also quite expensive and more appropriate for a concours car than for mine.  So I was left to find an appropriate tire that would restore the ride height.

To help in finding the right size, I found a website that computes tire specifications based on tire sizes.  Here are the results for the tire possibilities I was looking at.

Specification Sidewall Radius Diameter Difference
205/80-15 6.5 in 14.0 in 27.9 in 0.0%
205/75-15 6.1 in 13.6 in 27.1 in -2.8%
215/75-15 6.3 in 13.8 in 27.7 in -0.8%
215/70-15 5.9 in 13.4 in 26.9 in -3.8%
225/70-15 6.2 in 13.7 in 27.4 in -1.8%
235/70-15 6.5 in 14.0 in 28.0 in 0.1%
215/60-15 5.1 in 12.6 in 25.2 in -9.9%
225/60-15 5.3 in 12.8 in 25.6 in -8.2%
235/60-15 5.6 in 13.1 in 26.1 in -6.5%

As you can see, the original tire had a radius of 14.0" and all of the others got smaller.  By increasing the tread width for a given aspect ratio, the radius (ride height) also increases.  But you are putting more rubber on the road, making steering at low speeds more difficult.  You also run the risk of having the tire hit fenders, heat ducts or shields at full rebound.

70 series tires are quite common and are available in a lot of speed ratings.  75 series tires are less common and none (that I could find) had a very high speed rating.  So you are left with being able to go across speed bumps other than at a crawl or going very fast.  Given that I've never had my 330 above 110 mph, going very fast wasn't a high priority for me.  So I ended up looking for a 215/75-15 sized tire.  Since these are a tire size that is popular on larger American cars, most come in white-wall.  I wasn't particularly interested in having the white-wall exposed or mounted on the inside (where it could still be seen by anybody looking under the car), I limited my search to black-wall tires. 

I ended up with Michelin Symmetry P215/75R15.  The speed rating on this tire is S, which goes up to 112 MPH.  The radius is only .2" smaller, so the car will be back very close to the original ride height.  And no, just because they are M&S certified, I don't intend to drive in the snow or go 4 wheeling in the Ferrari.

It now been 10+ years and those Michelin tires are pretty hard, though not worn a lot.  One day, pulling out from my driveway with a damp road, the rear end spun out with just a slight acceleration.  Since I was having the wheels repainted, it was a good opportunity to get new tires.  Previously, the Michelins I bought were 215/75x15.  The car looked OK, but the spare barely fit in the tire well.  It took a little persuasion to get put away.  This time, I decided that I would get tires that were 205/75x15.  They will be a little smaller, so should fit the tire well better.  After looking over the current list of tires in that size, I ended up buying a set of Kumho Solus tires made in South Korea from Tire Rack.  These are a T rated tire, so a little better than the previous S rated Michelins.  Once the wheels were back from the paint shop, I had them mounted and balanced.  The tire shop didn't have the right adaptors for the balancing, but one of their techs figured out to use 2 cones, one on the inside and the other on the normal outside position.  This centered the wheel on the machine so it ran true.

   

Longstone Tyres in the UK has had Pirelli re-make the Cinturato CN72 tires in the original 205 VR 15 size (80 series).  However, they are quite expensive as the price of one is more than what I paid for the set of 5 Kumhos from Tire Rack.  Given that the judging in the US does not require a period tire pattern, just the same size as the original tires, the Kumhos meet that requirement.