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).
Comparison of road speeds using various tyre types and pressures for different car models.
| Identifier | ExFiles\Box 45\2\ Scan051 | |
| Date | 8th December 1927 | |
| ROAD SPEED WITH VARIOUS TYRES. BALLOON TYRES ARE AFFECTED MORE BY CENTRIFUGAL FORCE THAN HIGH PRESSURE TYRES. FOR THIS REASON THOUGH AT 30 M.P.H. THE BALLOONS GIVE A LOWER GEAR RATIO, AT 60 M.P.H. THE 6.75 AND 5.25" TYRES GIVE ABOUT THE SAME AXLE RATIO OVER ALL AS THE STRAIGHT SIDED THEY REPLACE. WE HAVE TAKEN A MEAN VALUE FOR ROLLING C'CE. ACTUAL VARIATIONS ENCOUNTERED. PRESSURE 50-30 LBS / □": 2% MANUFACTURING DISCREPANCIES. 1 1/2% VARIATIONS DUE TO NORMAL WEAR 2% VARIATIONS DUE TO VARIOUS AXLE WTS. 1% --- | DISCRIPTION OF TYRE | EFFECTIVE ROLLING CIRCUMFERENCE. | M.P.H. AT 1000 ENGINE R.P.M. | |---|---|---| | **40/50 CAR** | | **13 x 52 AXLE** | | 33 x 5 HIGH PRESSURE STRAIGHT SIDED 45 LBS / □" | 106.0" | 29.1 M.P.H | | 33 x 6.75 BALLOONS FOR 21" RIMS 35 LBS / □" | 103.25" | 28.3 M.P.H. | | 17MM x 50 MM. MICHELINS SPECIAL WHEELS 35 LBS / □" | 99.0" | 27.45 M.P.H. | | **20 HP. CAR.** | | **11 x 50 AXLE** | | 32 x 4.5 HIGH PRESSURE STRAIGHT SIDED 45 LBS / □" | 101.25" | 21.1 M.P.H. | | 31 x 6.00" BALLOON FOR 21" RIMS 35 LBS / □" | 100.0" | 20.8 M.P.H. | | 31x 5.25 BALLOONS FOR 21" RIMS 35 LBS / □" | 97.5" | 20.2 M.P.H. | | 6" BALLOON TYRES FOR 19.0" RIMS 35 LBS / □" | 92.0" | 19.3 M.P.H. | | **STUTZ CAR** | | **4.75-1 AXLE RATIO** | | 33 x 6.2 BALLOON GOODRICH | 98" | 19.5 M.P.H. | --- [Stamp] ROLLS-ROYCE LTD. [Stamp] 8 DEC 1927 | ||