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 speedometer drive revolutions, such as tyre size, inflation, and vehicle speed.
| Identifier | ExFiles\Box 59\2\ Scan064 | |
| Date | 4th September 1930 | |
| X4308 To R.{Sir Henry Royce} from EFC. c. Sg.{Arthur F. Sidgreaves - MD} c. Wor.{Arthur Wormald - General Works Manager} E.{Mr Elliott - Chief Engineer} c. Da.{Bernard Day - Chassis Design} Ey. PN.{Mr Northey} EFC2/AD4.9.30. X.4308. A.T. SPEEDOMETERS. In our EFC1/AD19.8.30 we mentioned the dependence of road wheel and speedometer drive revolutions per car mile upon :- (1) Tyre size (2) Degree of inflation (3) Speed. Actual variations which have been noted are as follows :- (1) In the case of Phantom II it is understood that the standard tyres at normal inflation pressure and slow or moderate speed give 609 wheel revolutions per mile, whereas the largest tyres in use give, in similar circumstances, 595 revolutions per mile, a reduction of 2.3% (2) Tests which have been made have shown that the road wheel revolutions per mile of Dunlop Silent Tread tyres on both chassis are increased by nearly exactly 3% by a reduction in tyre pressure from 40 to 20 lbs/sq.in. This result is presumably at slow or moderate speeds. At high speeds the effect would probably be less. 2% (say) (3) As mentioned in our report EFC1/AD19.8.30, tests appear to have shown that the road wheel revolutions per mile are decreased by 2½% at very high speeds as | ||