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).
Inaccuracy of speedometer readings following a test by 'the Autocar' on a 4¼ litre Bentley.
| Identifier | ExFiles\Box 82\4\ scan0317 | |
| Date | 5th June 1936 | |
| RM{William Robotham - Chief Engineer} S/W. Speedometer Readings. Cx{Major Len W. Cox - Advertising Manager}9/KW 5.6.36. You may have noticed that the Autocar, in their recent account of their test of the 4¼ litre Bentley made the following reference to the inaccuracy of our speedometers - " The speedometer proved to be 2.5 m.p.h. fast at a reading of 30 and 4 m.p.h. fast at a reading of 50, which margin was maintained approximately consistently up to the limit, the highest reading being 100-101 when the car was timed at nearly 95 m.p.h. " We, of course, doubted very much these statements and have deleted the whole sentence from the article which we are reprinting. I have in the meantime asked RC{R. Childs}, who gets every Trials Car speedometer tested by the makers against their master instrument, to ascertain exactly what margin they are providing and testing against, and I understand that they are working to the following figures :- at 30 m.p.h. the speedometer to read 31½ " 60 m.p.h. " " " 63 in other words up to this point the error to be 5% plus. At 100 m.p.h. the speedometer to read 107½, maximum total error 7½%. Will you please let us know if these are correct according to your requirements. We have always understood that the total margin of error was not in excess of 5%, and we ~~rather~~ take the view that 7½% is ~~somewhat~~ excessive. Cx.{Major Len W. Cox - Advertising Manager} LwB | ||