Hello,
is the formula for tire diameter   185 x % x 2divided by  25.4   +  14
let say 185.80.14                         |
|                           |           |
                                               widht
|                           |           |

ratio                       |           |
185 x 80% x 2
Convert mm    |
148 x 2 =296mm                                                    to
inches        |

|
296 / 25.4= 11.653
inches                                                      Wheel
diameter

11.653 + 14 inches =25.65 inches  Overall diameter

205 70 14

205x70% = 143.5

143.5 x 2 =287 mm

287 / 25.4 =11.299

11.299+ 14 inches = 25.299    Overall Diameter

On my westie i have 205 70 14
I didn't notice a lot a difference on the speedo betwen 185/ 80 to 205/
70
I think bentley shop manual say  in fourth gear you have 18 mile per
hour by
1,000 rpm     so at  4,000 rpm you should be around  70 or 72 mph
 I'm just betwen these two

 Denis B�lisle
Saint-Basile le grand
Qu�bec Canada
85 westie