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).
Tyre dimensions and wheel revolutions per mile for Phantom and 25 HP models, specifically concerning A.T. Speedometers.
| Identifier | ExFiles\Box 59\2\ Scan112 | |
| Date | 16th March 1931 | |
| To EFC. From Hs{Lord Ernest Hives - Chair}/Lid.{A. J. Lidsey} X6051a Hs{Lord Ernest Hives - Chair}/Lid.{A. J. Lidsey}2/MJ.16.3.31. X6051a/c PHANTOM AND 25 HP. A.T. SPEEDOMETERS. In EFC.1/AD13.3.31. it is stated that "wheels fitted with standard tyres at normal inflation pressure of 40 lbs/sq.in. make at slow or moderate speeds 609 wheel revs/mile". We would point out that the figure 609 applied to 33 x 6.75" (or 7.00 x 21") tyres which we ceased to fit as standard some considerable time ago. The present standard tyres are 7" x 20" Dunlop Silent Crossed Tread Fort Reinforced in the case of Phantom ll, and 6" x 19" Dunlop Silent Crossed Tread Fort Reinforced Tyres in the case of the 20/25 HP. car. These tyres have the following dimensions - (1) 7" x 20" Dunlop Silent Crossed Tread Fort Reinforced Tyres. | Pressure lbs/sq.in. | Effective Circumference at Slow Speeds (inches). | Wheel Revs/Mile at Slow Speeds. | |---|---|---| | 20 | 99.625" | 635.9 3.24% increase | | 25 | 100.5 | 630.5 | | 30 | 101.5 | 624.3 | | 35 | 102. | 621.2 | | 40 | 102.875 | 615.9 | Measured on rear of 19-EX. - Rear Weight 27 cwt. 7 lbs. (2) 6" x 19" Dunlop Silent Crossed Tread Fort Reinforced Tyres. | Pressure lbs/sq.in. | Effective Circumference at Slow Speeds (inches). | Wheel Revs/Mile at Slow Speeds. | |---|---|---| | 20. | 93.3125" | 679.0 2.45% increase | | 25 | 94 | 674.1 | | 30 | 94.75 | 668.8 | | 35 | 95.375 | 664.3 | | 40 | 95.5875 | 662.8 | Measured on rear of 18-G4 - Rear Weight 19 cwt. 2 qrs. 14 lbs. ( The normal rear tyre pressures on 20/25 cars is 35 lbs/sq.in) With regard to (3) of EFC1/AD13.3.31., we have no | ||