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 modifying speedometer scale indications to account for various factors.
| Identifier | ExFiles\Box 59\2\ Scan129 | |
| Date | 23th March 1931 guessed | |
| -9- speeds are rendered slightly higher than the true speeds even though the indications at the lower speeds are rendered very slightly low by this means. It must be remembered that in this we are always considering standard size tyres with normal inflation and one temperature (35°C.) of the instrument. If either the tyre inflation or the instrument temperature is somewhat low, the speed at which the instrument is driven will be correspondingly greater, therefore, as we have taken the extreme case, it is not likely that the speed indications at slow speeds will be appreciably reduced, but instead, that the effect would be still further very slightly to increase the speed indications at high speeds. Instead of basing the scale contraction on an approximate 3% effect at 90 miles per hour, an over-contraction might reasonably be effected based on an approximate figure of 5% at 90 miles per hour. This would mean that the figure 90 must be placed against the 85 miles per hour division of an unmodified or proportional scale. do the unmodified deflection corresponding to 90 miles per hour is then still 90 divisions but d the modified deflection corresponding to the same speed is 85 divisions, whence, in this instance, we have the numerical equation :- 85 = 90 - k.90^2 or k = 5/90^2 = 1/1620 = .0006 (approximately) Therefore, finally the rule for the placing of any figure representing the modified miles per hour reading is d = do - .0006 do^2 From this relation we may similarly calculate, for every value of do i.e. the miles per hour reading on the original proportional scale, the number d of divisions on that proportional scale at which the figure do should now be placed to produce the scale contracted again in accordance with the above developed law, but by the augmented amount. Such a table calculated for every 5 miles per hour increment of speed is given below :- | ||