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).
Speedometer calibration calculations for a Bentley 3 1/2 Litre chassis based on different tyre and axle configurations.
| Identifier | ExFiles\Box 165\4\ img219 | |
| Date | 27th July 1934 guessed | |
| 81807 BENTLEY 3 1/2 LITRE CHASSIS SPEEDOMETER CALIBRATION Effective tyre circumference (found by actual trial) = 90.375" with India tyres 35 lbs per sq.in. 89.375" " Goodyear " 35 " " " 90.875" " Dunlop (new silent tread) " Therefore wheel revs per mile = 1760 x 36 / 90.375 or 89.375 or 90.875 = 701.1 for India tyres. = 709.0 for Goodyear tyres. = 697.2 for Dunlop (new silent tread). Assuming an axle ratio of 10/41 Prop. Shaft revs per mile = 2870 India. 2907 Goodyear. 2858 Dunlop (New silent tread). Gear ratio prop shaft to speedo drive = 2/16 x 20/9 Hence speedo drive revs per mile = 797 India tyres 807 Goodyear. 794 Dunlop (New silent tread). ------------------------- 11/41 axle ----------- 724 India tyres 734 Goodyear. 11/43 axle ----------- 759 India tyres. 762 Goodyear. Hs{Lord Ernest Hives - Chair}/Wst. | ||