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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).
Test report on A.T. speedometers No. C.296 & C.278, detailing performance and suitability on a 22-EX vehicle.

Identifier  ExFiles\Box 59\2\  Scan022
Date  15th April 1930
  
FOR FILES.
XU 308
Hs{Lord Ernest Hives - Chair}/AJL.15.4.30.

A.T. SPEEDOMETERS No. C.296 & C.278.

The above speedometers were tested on 22-EX.
7" x 20" Dunlop Silent Tread tyres were used, these have an effective running circumference of 102.5" at 35 lbs/sq.in. pressure. (i.e. Wheel revs/mile = 618.1).
This gives a speedometer drive revs/mile figure of :-

618.1 x 41/11 x 2/19 x 26/8 = 790.

The speedometers are arranged to be correct for a figure of 800 speedometer drive revs/mile, (they should be 1.26% slow on this car). Tested on the road the results are as follows, (both speedometers give approximately the same result).

Speed indication :-

At 20 M.P.H. speedometers are 1.3% slow.
" 30 M.P.H. " " " 1.25% slow.
" 40 M.P.H. " " " 1.7% slow.
" 50 M.P.H. " " " 1.4% slow.

Mileage recorder 1.675% low.

These speedometers are not suitable for a standard production car when fitted with 7 x 21" tyres. They are only nearly correct on 22-EX. because the servo gears are old type. The ratio is altered on standard production. The
  
  


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