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).
Wheel and tire dimensions, performance data, and speedometer calibration.
| Identifier | ExFiles\Box 19\3\ Scan215 | |
| Date | 17th March 1930 | |
| To Bv. c. Rm{William Robotham - Chief Engineer}; c. Srn. X777? Oy3/DM.{D. Munro}17.3.30. Wheels and Tires. Burton cables - Rolling diameter 19 x 7 U.S. Royal Cord Tire - Front 32" Rear 31½" Free diameter 33½" Have run 2000 miles on 69-WJ with no complaints(shimmy etc) Standing height under load approx. 15½". These are 6 ply standard section tires, whereas we believe the tendency will be to use heavier section casings on cars of our weight. We therefore suggest (and have made ourselves) the following assumptions - Rolling diameter 32" Circumference 100.5" Turns per mile 630 [handwritten note: wheelhouse] Free diameter 34½" (for wheelhouse clearances etc) Standing height 15¾" (for appearance and road clearance). Without deliberately "cheating" on the speed we believe it is well to make the speedometer indication from 1% to 2% high on the above figure of 630 turns per mile, to allow for "special" tires which are always overweight, and are very popular with chauffeurs and Trans-Continental tourists, and to avoid any danger of over-shooting the mark when following an unknown road from route-book directions. Mr. A.Y. Gowan, who happens to be here to-day, confirms from his personal experience as owner of 9 R.R. cars that this is correct practice. cy. Maurice Olley | ||