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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).
The testing of a new speedometer at Brooklands, detailing its accuracy and the effects of tyre diameter.

Identifier  WestWitteringFiles\U\August1930-November1930\  Scan001
Date  14th August 1930
  
To R.{Sir Henry Royce} from Hs{Lord Ernest Hives - Chair}/Bm
c.c. SG.{Arthur F. Sidgreaves - MD}
c.c. Wor.{Arthur Wormald - General Works Manager} Da.{Bernard Day - Chassis Design}
c.c. E.{Mr Elliott - Chief Engineer} By.{R.W. Bailey - Chief Engineer}

ORIGINAL

Hs{Lord Ernest Hives - Chair}/Rm{William Robotham - Chief Engineer}2/AD14.8.30.

X4308

SPEEDOMETERS.

We recently carefully checked speedometer No. D.668, a new speedometer taken from production, on Brooklands track.

We checked the mileage recording instrument over a distance of 27 miles and found that it read 2½% fast at an average speed of 60 M.P.H. We checked the M.P.H. of the instrument and found that it read 2% fast at 75 M.P.H.

We afterwards handed the instrument to EFC. to test on his rig and he recorded that against driving spindle revolutions it read 5% fast both for mileage and M.P.H. It would therefore appear that the increase in effective tyre diameter with speed causes the speedometer to read slower than might be anticipated.

It is clear that this 'slowing down' of the speedometer reading is greatest at maximum speed where it is most desirable that the speedometer should not read slow. For mileage, which is recorded at about 40 M.P.H. the reverse results.
  
  


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