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).
The calibration of an A.T. Speedometer for Chassis 18-EX due to non-standard tyre size.
| Identifier | ExFiles\Box 19\5\ Scan023 | |
| Date | 6th December 1929 | |
| To Py. from EFC. " Hs.{Lord Ernest Hives - Chair} " Wcr. C. Hs{Lord Ernest Hives - Chair}/AJL. x8992. EFC2/AD6.12.29. X.4308. A.T. SPEEDOMETER FOR CHASSIS 18-EX. We have heard nothing in respect of our EFC1/AD22.11.29. We have discussed the matter with Mr. Dodd and it transpires that owing to the use of rather smaller tyres than usual and the standard speedometer gearing, this instrument will be driven at 837 revolutions per mile. The instrument itself being one of our present standard 765 revolutions per mile instruments, the mileage error will be over 9% high. At Mr. Dodd's request we have agreed to set the instrument for speed indications, 4% high at 30°C. i.e. speed reading at this temperature will be such as to correspond correctly to 804 revolutions per mile thus making the instruments internal error at this temperature that of the speed reading low in relation to the miles indicated in the proportion of 765 to 804. We consider that the circumstances in which an error of this kind becomes necessary should not arise. The actual cold setting of the instrument is such as to read 46.0 miles per hour at 20°C. when driven at 600 revolutions per minute. E.F.C. EFC. | ||