Tires: Speed Rating, passenger car tires, light truck tires

Question
How to calculate the speed rating of a tire if its not indicated on the tire.  I have a few examples
10-15/B
185R14
225/50R18
37x12.50R16.5/E
ER78-14/B
LT245/70R17/C

Answer
A,

If the speed rating is not indicated on the sidewall, then they are NOT speed rated.

Speed rating is a European based system to meet the need for tires where the speed of the vehicle is above "normal" highway speeds. Tires without a speed rating are exactly that - without a speed rating.

This means that the tire is capable of the legal limit - and that's it - unless otherwise indicated, such as the letters "NHS", "MH", or the like.  

However, most unrated passenger car tires are capable of achieving an S rating if they were tested.  Light Truck tires (in your examples 10-15, 37X12.50R16.5, LT245/70R17) the speed rating could be quite lower.  I've seen as low as Q for normal highway LT tires.

BUT - and I want to emphasize this - the rating is not a size dependent sort of thing.  Any of the sizes you mentioned could come in a Y rating (300 kph / 185 mph) or in an A1 rating (5 kph / 3 mph).

The manufacturer determine what the speed rating is and he marks it on the sidewall.  If it isn't there, it probably mean he didn't test it.  You can't draw any other conclusions from that.

But, you have to be very careful if the speed rating isn't indicated.  For example - many winter tires aren't speed rated, and using them as you would a normal tire in the summer would be foolish.

So the obvious question now is - Why are you asking?