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).
Analysis of factors causing instrument reading errors for speed and mileage, specifically related to tyre size and temperature.
| Identifier | ExFiles\Box 50\4\ Scan339 | |
| Date | 6th December 1929 | |
| EFC1/AD6.12.29 contd. -2- In the circumstances above outlined an instrument could never read low either on miles or speed because of (1) The fact that the instrument revolutions per counter mile will always be less by varying small amounts than the drive revolutions per car mile. (2) The use of tyres which are always smaller by varying small amounts than the largest tyres on which the revolutions per car mile is based. (3) In the case of the speed reading only, the fact that the instrument will mainly be at a lesser temperature than 30°C, which fact will still further increase the speed reading by some amount. This is referred to in (c) above, and, we believe, is already arranged with the manufacturers not to exceed 4% between 30°C. and 18°C. We do not agree with the present position of these instruments by which they are made to read both on miles and speed, with what is in our opinion, an unduly high error. In the circumstances above outlined the error would always be slightly high, as desired, but by a reasonable amount only. EFC: | ||