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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).
Letter from Vauxhall Motors discussing tyre pressure limits and typical tyre sizes for American passenger cars.

Identifier  ExFiles\Box 170\3\  img183
Date  3rd January 1939
  
oy.

Vauxhall Motors Ltd
CONTRACTORS TO THE ADMIRALTY, WAR OFFICE & AIR MINISTRY.

TELEPHONE
NUMBER 2600 LUTON
LUTON. BEDS.
TELEGRAMS
CARVAUX, TELEX, LUTON

WHEN REPLYING PLEASE QUOTE
→ OUR REF 289/MO/HMD.
YOUR REF Rm{William Robotham - Chief Engineer}/Gry.{Shadwell Grylls}10/MH.{M. Huckerby}

3rd January 1939.

Mr. S.W.Grylls.
Rolls Royce Ltd.,
DERBY.

Dear Grylls,

Yours of December 22nd.

Limit pressure on 4 ply tyres is based on the chances of blow out on long runs in hot weather.

Actually the 28 lb. inflation limit on 4 ply tyres is not strictly adhered to and 4 ply rear tyres are frequently inflated to 30 and 32 lbs. cold.

For an axle load of 1150 lbs. at the rear with three passengers, I should use a 6½" - 16" four ply tyre at 32 lb, allowing slight overload, if I wanted to save weight or money, or a 7" - 16" four ply tyre at 27 lbs for maximum comfort.

In actual practice American passenger car tyres are :-

5½ - 16 4 ply for smallest cars.
6 - 16 " for Chev.Master, Ford, small Pontiac, etc.,
6½ - 16 " for the smaller eights and larger sixes.
7 - 15 or 7 - 16 " for the "upper medium" cars and some "lower medium"
7½ - 16 6 ply for the "mammoths"
8¼ - 16 " for super-mammoths like Packard 12.

These tyres are available all over the world, which is a strong argument for using them.
  
  


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