From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Speedometer accuracy, the effect of larger tyres, and calibration standards.
| Identifier | ExFiles\Box 160\3\ scan0192 | |
| Date | 14th February 1939 | |
| - 2 - car, I sent it to A.T.'s this morning, and they told us that at 60 m.p.h. it was showing a fraction fast - about ¾ of a mile - which of course is negligible. From 60 - 110 it was showing fractionally slow against their master instrument. Thus, on this particular car our accepted margin in favour of the car of about 5% at the higher speeds does not exist. The speedometer people put this down to the fact that we have introduced larger tyres and have not made a corresponding allowance in the calibration of the speedometer dial. Can you please tell me if that is a correct statement. I certainly think we are quite within our rights in showing a margin up to 5% in favour of the car at the higher speeds, and that we ought not to tolerate any error which is against the car's performance. I am having another Bentley checked up this afternoon, to see what results we get here as confirmation or otherwise of the statement that the A.T. people have given us this morning. I was under the impression that the larger tyres did not materially affect the gear ratio, and certainly not to the extent that A.T's are suggesting. There seems to be a wide divergence of opinion as to what the speedometer really should show, and I would be glad if you would let me know what is the present standard of calibration, and the results from the point of view of the driver. Cx.{Major Len W. Cox - Advertising Manager} huel | ||