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).
Test report on an A.T. speedometer for a 20 HP chassis, analyzing its accuracy and temperature-related errors.
| Identifier | ExFiles\Box 50\4\ Scan336 | |
| Date | 2nd December 1929 | |
| X4308. To Ry. from EFC. " Hs.{Lord Ernest Hives - Chair} " Wor.{Arthur Wormald - General Works Manager} EFC7/AD2.12.29. X.4308. A.T. SPEEDOMETER ON 20 HP. CHASSIS. An up-to-date 720 revolution per mile instrument D.500. 720. 11/50 has been checked for gear ratio to counter readings at 600 r.p.m. exactly per hour. The instrument was found to be correct in regard to the 720 spindle revolutions per counter mile. The speedometer drive revolutions per mile on the 20 HP. standard chassis fitted with standard tyres are 750. Mileages will therefore read about 4.1% high. The speed indications of the instrument at 600 r.p.m. exactly with the correspondingly correct drive revolutions per mile were found to be :- | Temperature (°C) | Miles per hour indicated at 600 r.p.m. | Corresponding correct revs. per mile | | :--- | :--- | :--- | | 20.0 | 51.3 | 702 | | 22.0 | 50.9 | 707 | | 24.0 | 50.4 | 714 | | 26.0 | 50.2 | 716 | | 28.0 | 50.0 | 720 | | 30.0 | 49.9 | 721 | | 32.0 | 49.7 | 724 | shewing the instrument to be correct in itself at 28.0°C. with an error of 2.7% between 20° and 30°C. This temperature error is reasonable. On the chassis the speed error cold (20°C) will be 6.8%. and warm (30°C) will be 4.0%. EFC. | ||