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 when fitting low-pressure tyres to a Silver Ghost chassis.
| Identifier | ExFiles\Box 53\3\ Scan033 | |
| Date | 16th March 1927 | |
| To HY.{Tom Haldenby - Plant Engineer} from Hs{Lord Ernest Hives - Chair}/Rn.{Mr Robinson} c. to C. +4565 Hs{Lord Ernest Hives - Chair}/Rn{Mr Robinson}4/LG16.3.27. LOW PRESSURE TYRES ON SILVER GHOST CHASSIS. With reference to EY12/G15.3.27. You state that the speedometer will read 3 1/2% higher with balloon tyres than with the high pressure tyres. This is only true at low speeds - at high speeds the circumference of the low pressure tyre increase due to centrifugal force more than that of the high pressure and their effective diameters become the same. A note on this is given in our reports on 'Fundamentals of Car Performance' - Tyres. We therefore do not consider any alteration is necessary to the speedometers when changing over. Hs{Lord Ernest Hives - Chair}/Rn.{Mr Robinson} | ||