Below is the contents of the paper I wrote to Avon a couple of years ago. Enjoy but bear in mind I worte it from a GTD owners perspective! And it is my own opinion and not of any company or club.
There are 400 GTD chassis’ world-wide and the standard tyre sizes were 265 50 VR 15 on a 9 inch rim at the rear and 215 60 VR15 on a 7 inch rim at the front. Tyres made by Goodrich, the Comp TA 2. This tyre was just about ok until a driver got enthusiastic when it would quickly overheat and lose grip. Our cars could out perform the tyre, especially on a track day.
Other than these standard rim sizes a very popular rim size choice now is 10 inch at the rear and 8 inch at the front. I would suggest that the split is now 50/50 between these two pairs of rim sizes.
Since Goodrich stopped making the 265 50 15 rear tyre, owners have tried all types of tyre to get a good result. These have ranged from using HR rated tyres with a size of 305 50 15, Comp TA to Pirelli P7 275 55 Z 15 and Dunlop Post Historic 6.00 1200 15 or 5.25 1000 15. The latter are questionable over road legality.
The fronts are easier to get but to get a matched pair of sizes front and rear has not always been possible, with owners mixing makes, speed ratings or compounds! Not always a good idea.
There are two main types of usage namely road cars and those that get put on a track for track days or competition. I believe that the two types of use will have different size requirements although some owners will overlap into both categories.
For road use the owners will want a tyre that looks the part of a sixties vintage tyre. This means a tall tyre. To fit our cars at the rear anything over 27.5 inches is very likely to rub the wheel arch on bump. Your thoughts of a 275 60 15 and a 255 65 15 come to 28 inches by my calculations and so would not fit. A range of road sizes would be between 275 50 15 to 305 50 15. To me a 295 50 15 is the ideal road size for the rear.
At the front the risk of tyres fouling the inner wheel arch linings is even worse. But it is known that a wider tyre really helps the car turn in. One owner used tyres up to 245 50 15 at the front. This was right at the limit of fitting. A 25 inch overall diameter would seem optimum for road use. This translates to 215 60 15, 225 55 15, 235 55 15 or 235 50 15. I think the 245 50 15 is too wide and would cause most owners problems with fouling. To match the 295 50 15 above I would think an ideal road size is 235 55 15.
In competition on the hills and in sprints an overall smaller diameter helps. Often Avon slicks are used and the sizes are, I think, 23.5 8.5 15 at the front and 25 11 15 at the rear. The small diameter rear really helps in acceleration off the line. You mentioned a 335 35 15. I would be concerned whether the width would fit in the arch without modification. Owners are generally reluctant to modify the body work on their 40 replicas. Especially once built. The 11 inch slick just fits. Just! The side wall is much squarer on the slick than on a road tyre so I am not sure how the two tyres would marry up in cross section. The 12 inch Dunlop post historic is 12 inches wide at its maximum i.e. the bulge of the side wall but has an approximate 10 inch tread foot print. The problem to me with this tyre is its weak side wall in that when the car is leaning round a corner, the roll of the tyre makes the tyre wall very close to suspension bits with the risk of slicing the tyre open. Not good!
If it is raining we would want a larger diameter tyre to reduce the effect of our powerful engines as the back end does break loose easily in the wet. The road sizes would be good then. If you were able to create a size that matched the slick sizes then that would be a good compromise for wet and dry usage. Not all competition guys have spare sets of rims due to the high cost for wheels.
KVA and GTD are no longer in production but Tornado is with CAV starting distribution in the UK this year. There are other manufacturers around the world with RF in Australia being the biggest of the names I know. ERA and Holman Moody do produce cars but in even lower numbers than the other manufacturers.
There has been lot s of discussion on the GT40s forum (
www.gt40s.com) about tyres and a lot of owners are looking to 16 inch and 17 inch rims to get round the tyre size problems. It would be very good if you could set a new standard for 15 inch tyres as that is what the GT40 replicas need badly to look right. You may also wish to go to
www.gtd40club.co.uk. Downloadable in car video and pictures that show what we get up to.
With most owners doing relatively low mileage per year say up to 5000 miles, often much less, tyre compounds can be quite soft. The softest road compound would need to last a track day without overheating.
I hope this information is of help and I would be very keen to see you produce some good sized tyres for our cars as I think that if you do, Avon may be the new GT40 replica industry standard