my 78 van states it wants L78-15 tires. the diameter from the charts on internet say the diameter is 29 inches.the P235/75/R15 diameter is supposed to be 28.9 inches. I measure 27 inches. What is wrong? I have checked the speedo cable.swapped the speedo with another same type. I have timed the speed and for a 60 second mile,the odometer indicates 0.8 mile;the speedo
reads 50 mph. thank you for any help. Tony

 

Just a curious thought, is the rear gear the same one that originally came with the truck?