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 Mr. Minchin of Messrs. Peto & Radford concerning a speedometer gearing and reading discrepancy.
| Identifier | ExFiles\Box 50\4\ Scan314 | |
| Date | 28th October 1929 | |
| X4308. EFCA/AD. 28th October, 1929. Mr. Minchin, Messrs. Peto & Radford, 50, Grosvenor Gardens, London, S.W.1. Dear Sir, Speedometer C.156. 700. 12/50. Your speedometer, although marked 700 is actually geared in itself to 675 drive revolutions per counter mile. I am informed that the road wheel revolutions per mile of the 20 HP. chassis assuming 6" x 19" tyres, are 660 so that the speedometer drive revolutions per mile are 660 x 50/12 x 1/4 = 687.5 The mileage reading, based on the above figure of 660, would therefore be 1.85% high. N.B. I cannot myself verify the figure of 660 which depends on the tyres. As regards speed readings, the instrument to be correct in itself should read 60 miles per hour at 675 revs per minute, or 53.3 miles per hour at 600 revs. per min. We find it to read (when cold) 50.8 miles per hour at this speed so that the instrument is (when cold) 4.7% slow to its own miles reading. When hot it will be worse. It looks as if there is some error concerning this instrument, because if we had assumed the 700 marked on the instrument to be the correct revs. per mile, then the speed reading at 600 r.p.m. should be 51.4 to be correct, which is very nearly what it is. The whole thing suggests that the speed needle has been erroneously set to agree with 700 drive revs. | ||