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).
Calculations for speedometer gear ratio and revolutions per mile, accounting for tyre pressure and speed effects.
| Identifier | ExFiles\Box 59\2\ Scan117 | |
| Date | 24th March 1931 | |
| EFC1/AD24.3.31. contd. -2- The effect of a reduction of inflation pressure from 40 to 20 lbs/sq.in. appears from tests to be to increase this figure to 636 (3.24% increase) according to a linear law with falling pressure. The effect of speed from dead slow to 90 miles per hour probably reduces this figure to 598 in accordance with the 3% which we know to be approximately the amount of this effect. The gear ratio from the road wheels to the speedometer drive is 41/11 x 2/18 x 26/8 = 1.346 hence the number of revolutions per mile (at slow speed and normal tyre inflation) of the speedometer drive is 616 x 1.346 = 829 There is no intention of making any allowance for the effect of deflated tyres in increasing both the mileage and speed readings, but the whole intention is to correct⊗ as reasonably as possible the effect of speed in reducing the revolutions per mile of wheels with normally inflated tyres (40 lbs/sq.in.). In accordance with the principles explained on our general sheets, it is necessary to utilise an instrument which is made, not for 829 spindle revolutions per counter mile but (see page 6 1/4 of those sheets) one made for 829 x 44.27/45.00 = 816 revolutions per mile. 816 therefore, is definitely the figure for the instrument required for the Phantom II in order that the mileage shall be correct on the average. At present, standard instruments are ordered for 785 revolutions per mile -- we ourselves have advocated a figure of 794, see our EFC2/AD3.9.30. ⊗ in fact slightly over-correct | ||