Thanks to everyone who has contributed to my question. Kumho 185/80's have been fitted a) because they are readily avilable here in OZ and b) they have performed well on my TA21 DHC. I have concluded that there is no direct Radial equivalent to the original 6.40 Cross Plies. Last week I climed into the loft and retrieved an original cross ply and compared it with the radial 185/80. The cross ply stands between 15 to 20 mm higher than the radial. The rim to tread is 14mm higher and when under the load of the car, the cross ply deforms about 14mm whilst the radial deforms between 20 to 25mm. The softer wall of the radial also suggests that it will deform even more under actual road use conditions, hence the revolution dimension each turn will be considerably different to that of the cross ply. Add to this the tread width difference --- the radial is about 40mm wider on the road than the cross ply, means that this greater surface area will give significantly better braking, but cause greater strain when attempting to park the car if the vehicle does not have power steering. It is possible too that the required tyre pressures in the radials may vary between the brands. I usually run at 36psi but when I fitted a soft wall Pirelli to one of my cars, the agents insisted that I run these at 52psi because of the materials used in the tyre. Because of all the above, I now use my GPS to determine the correct speed of the car. Posted by pdadd 23/03/2013 01:48:49