850/50R38

tire  overall diameter - 1810mm
tire  overall width    - 710mm
wheel overall diameter - 1016mm

LEGO solution
tire  overall diameter - 114,8mm
tire  overall width    - 53mm
wheel overall diameter - 64mm

In order to achieve 114,8mm overall diameter from Powerpuller tire, 
you have to put one 94,8 X 44R Baloon tire inside it.

Important thing is that you put dish inverted 10 x 10 at both sides of tire 
between powerpuller abd 94,8 X 44R Baloon tire and then press them using dish 8 x 8, 
axle 10 treaded and axle nuts

Parts list:
1 x http://www.bricklink.com/catalogItem.asp?P=32298
1 x http://www.bricklink.com/catalogItem.asp?P=54120
2 x http://www.bricklink.com/catalogItem.asp?P=47675
2 x http://www.bricklink.com/catalogItem.asp?P=3961


for any commants, questions please contact me at
ipsa.igor@gmail.com