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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).
Summary of fuel consumption and performance tests comparing Napier, Silver Ghost, and Silver Rogue cars, with calculations for windage effects.

Identifier  ExFiles\Box 20\6\  Scan015
Date  25th October 1911
  
(3)

PFRS. No. 527. 25.10.11 (Sheet 3 of 3)

Time taken to consume one gallon on the road, at 20 m.p.h. is about one hour.

Therefore, the horse power absorbed (one H.P.) by the extra windage area (9.85 sq. ft.) at 20 m.p.h. represents very roughly .092 galls. of petrol per hr.

If windage of Silver Rogue had been cut down at 10.4 sq. ft. the consumption would have been reduced to about 22.1 m.{Mr Moon / Mr Moore} per gall. or 45 ton miles per gallon.

SUMMARY.

Car.
Miles per gall.
Ton miles per gall.
Distance run
Speed at 1000rpm.
Windage.

Napier
19.85
42.57
799 ms.
37 1/4 mph.
10.4 sq. ft.

Silver Ghost
20.8
40.8
420
37.88

Silver Ghost
23.25
45.8
46 1/4
37.88

Silver Rogue
20.1
40.98
2000
39.46
19 3/4

Silver Rogue corrected for windage.
22.1
45
2000
39.46
10.4

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