 SUSPENSION AND THE ROUND BITS
 185 Tyre section width in mm 70 Relation of section height to section width in % R Construction (R = Radial) 13 Rim diameter in inches 86 Load index (Max load, 86 = 530kg) H Speed symbol (H = 13Omph) M+S = Mud and Snow (On a Seven?)

 Rim markings (Drop centre rims). Example: 6J X 13 H1
 6 Rim width in inches J Rim flange height X One piece rim 13 Rim diameter in inches H1 Number of humps (used to keep tyre in place

 A secondary reason for changing tyres was the possibility to alter the cars overall gearing by changing the rolling radius of the tyre. The mph per rpm can be found with the following calculation; Engine rpm multiplied by 60 (revs per hour), divided by gear ratio divided by axle ratio = Number of wheel revolutions per hour. Wheel revs per hour multiplied by rolling circumference = inches travelled per hour. Inches per hour divided by inches in a mile (63,360) = MPH Therefore, the information needed is engine rpm, axle ratio, gearbox ratios and tyre rolling circumference. For my car that was; Max engine rpm = 6,800 (although I regularly go to 7,400!) Axle ratio = 3.636:1 Tyre rolling circumference for 185/70 13 881 revs per mile (taken from data book) = 72 inch circumference or Tyre rolling circumference for 185/60 13 954 revs per mile (taken from data book) = 66.5" circumference Giving the following gear speeds for my car with 4 speed Escort Sports box and both 70 and 60 profile tyres:
 Gear Ratio Speed Speed 185/70 185/60 1st 3.337 38mph 35mph 2nd 1.995 63mph 58mph 3rd 1.418 89mph 83mph 4th 1.00 127mph 117mph 