Tires: trailer tires, rim width, trailer tires

Question
I asked a question the other day about tires for my trailer. While what I
ultimately need to know is if they will fit, what I intended to ask was
regarding the difference between the two tire sizes. What is the physical
difference (dia. in inches perhaps) between a b78-13 (175/80-13) and a
c78-13 (185/80-13)? If I can understand this difference I can determine if
they will fit.
Thanks again,
William

Answer
William,

Thanks for posting back.  I hope realize I get a lot of the "Will it fit?" questions - and I just don't have access to charts for the thousands of vehicles that are out there.

But here is the info you need:

Size:    Diameter    Width    Rim width range   Max load / pressure
B78-13ST   24.0"      7.1"    4 1/2" to 6 1/2"  1075# @ 35 psi (LR B)
                                               1325# @ 50 psi (LR C)

C78-13ST   24.5"      7.3"    5" to 6 1/2"      1135# @ 35 psi (LR B)
                                               1390# @ 50 psi (LR C)

ST175/80-13 24.0"     7.0"    4 1/2" to 5 1/2" 1100# @ 35 psi (LR B)
                                              1360# @ 50 psi (LR C)

ST185/80-13 24.7"     7.2"     4 1/2" to 6"    1200# @ 35 psi (LR B)
                                              1480# @ 50 psi (LR C)

Notice that these are 4 different sizes and each has a pair of Load Ranges (LR) with a different load carrying capacity.  It is important that you know what you are starting with so you do not undersize the tire.  Tire failures are sometimes tragic!