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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 calculations and testing for a 10,000-mile test on car 23-EX.

Identifier  ExFiles\Box 59\2\  Scan002
Date  17th January 1930
  
COPY. X4308 17.1.30.
EFC. from Hs{Lord Ernest Hives - Chair}/AJL.

X.4308. SPEEDOMETER FOR 10,000 MILES TEST ON 23-EX.

This car is to have 20" wheels fitted with
Dunlop 32 x 6.75" Triple Stud tread Fort Reinforced tyres which
give a figure of 622 revs/mile.

The speedometer drive revs/mile corresponding
to this are :-

622 x 41/11 x 2/18 x 26/8 = 836.1 revs/mile (for
standard 11/41 rear axle)

We are handing to you the speedometer off this
car in order that you may check and correct it if necessary,
and also carry out your first test to ascertain if
any loss of magnetism occurs after considerable usage.

HS{Lord Ernest Hives - Chair}/AJL.
  
  


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