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).
Letter to A.T. Speedometer Co. Ltd. regarding the calibration of speedometers for different axle ratios.
| Identifier | ExFiles\Box 50\4\ Scan247 | |
| Date | 31th August 1928 | |
| X4308 EFC {E. Fowler Clarke - Electrical Engineer} 2/T. 31st August, 1928. Messrs. The A.T. Speedometer Co.Ltd., 20, Avonmore Road, West Kensington, W.14. Dear Sirs, Referring to the last paragraph of yours of the 29th inst. with reference to the experimental small dial type speedometers, the information you require still stands as given in our EFC {E. Fowler Clarke - Electrical Engineer} 1/T. of the 17th March last, i.e. for the 14/52 axle ratio 800 revs.per mile, and for the 15/52 axle ratio 747 revs.per mile. You have provided us with a 14/52 800 revs.per mile instrument and now we are applying to you for a 15/52 which, to be correct, should read 747 revs. per mile, but it appears doubtful whether you can arrange your instrument to such an awkward figure, and possibly that is the origin of the 742 revs.per mile which you mention. We should be interested to know what is the nearest figure to 747 which you can give, it being desirable to have this figure lower rather than higher, so that the error in the mileage reading will be positive rather than negative. In each case it is understood, as was the case in the 14/52 800 revs. per mile instrument, which we have just tested, that the speed needle reads correctly at a temperature in the neighbourhood of 30°C, and consequently has a positive error at lower temperatures. The correctness of the m.p.h. indication at 30°C. should be on the basis of driving the instrument at 747 revs. per mile, i.e. when the instrument is driven at 600 revs. per minute it should read 48.2 m.p.h. Yours faithfully, FOR ROLLS-ROYCE LIMITED. | ||