Royal Mail tyre specs

Model Front Wheel Rear Wheel
     
Rod Brake bike 26" x 1.5" 26" x 1.5"
     
RM92 26" x 1.5" 26" x 1.5"
     
Millenium 26" x 1.75" 26" x 1.75"
     
Mailstar 24" x 1.75" 26" x 1.75"

 

Dave Peckham from Village Bicycle Project (VBP) says:

I started seeing those postie tires everywhere, First in Lome, Togo, then Hohoe, Accra, Kumasi, Bolga.  My recollection is they were 26 X 1 1/2.