Sir, It is surprising what singular facets of the motoring scene can come to light even after a good many years’ driving experience. In my own case recently, after replacing the front tyres of my car with Goodyear G8, “Super Cushion” 5.90 x 15 in. crossply covers I immediately noticed that the new-tyres were appreciably smaller than their predecessors – crossply 5.90 x 15 in. India “Autoway”. Having retained the discarded tyres a comparative measurement disclosed that the worn India covers still possessed a rolling circumference of 81 in. whereas the new Goodyears, full tread thickness and all, gave a reading of but 79½ in. Doubtless this discrepancy will increase with wear. Neither make of tyre incidentally, has any seeming claim to low profile or other special side-wall variation.
In correspondence with the company Goodyear confirm that their G8 5.90 x 15 in. covers are smaller than the equivalent India tyre but assure me that their product complies with standards laid down for the industry in the UK and most European countries. They also cite “easing growth” as possibly increasing the dimensions of an old tyre although 1½ in. here seems pretty fantastic to the layman at least. Belonging to a one make car club I have mooted the subject of tyre sizes and a fellow member has found that Firestone covers are similarly under-sized whilst a number of other makes are, again, up to 2 1/4in. larger. The following are some of the queries that come to mind:
What is the possible effect on a car’s stability should a new, comparatively large size cover find itself fitted opposite a worn, smaller “equivalent’?
Does this discrepancy in sizes extend to other gauges of tyre. both cross-ply and radial?
Will the smaller cover prove as durable as its larger counterpart ?
Does the manufacturer of relatively small tyres obtain a bonus via the number of covers he is able to produce from a given amount of rubber mix ?
Why is it necessary to produce comparatively smaller tyres? (One excludes low profile and other special designs from this question.)
It would be re-assuring if an independent expert could dispose of these misgivings.
Thus, all other things being equal (and there is no reason to suppose otherwise) it appears pointless if not downright foolish for a motorist to purchase tyres that are dimensionally inferior and not necessarily cheaper than alternatives that may be had for the asking.
K. H. Daws