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).
Factors affecting the accuracy of Bentley speedometers, including tyre effects, temperature, and wear.
| Identifier | ExFiles\Box 82\4\ scan0318 | |
| Date | 9th June 1936 | |
| W/S - Cx.{Major Len W. Cox - Advertising Manager} X 200 Hs{Lord Ernest Hives - Chair}/Gry.{Shadwell Grylls}8/KW.9.6.36. Re Speedometers. With reference to your Cx.{Major Len W. Cox - Advertising Manager}9/KW.5.6.36, we have gone into the matter of the optimism of the Bentley Speedometers. The India tyre people tell us that the centrifugal effect on the tyre at 100 M.P.H. will increase its running radius about 2%. The speedometer when heated to 35°C. reads 1½% slower than at 10°C. Allowing for these two conditions, 3½% of the 7½% you quote at 100 M.P.H. must be subtracted. This means that speedometers will actually only be 4% fast at 100 M.P.H. and 5% at lower speeds. We do not consider this an excessive amount. At high speeds on a bumpy road, the speedometer may appear unduly optimistic owing to spin of the rear wheels. Also when all the tread is worn off the rear tyres it will read an additional 2½% high. We hope this answers the questions in your memo. Hs{Lord Ernest Hives - Chair}/Gry.{Shadwell Grylls} | ||