GarageCalc 
Reading the full service description
On a tire marked 225/45R17 94V, the 94 is the load index (1,477 lb per tire) and
the V is the speed rating (149 mph). Together they form the
service description, and both must meet or exceed what your vehicle placard specifies. When
changing tire sizes, compare capacity, not just dimensions: check the size math with the
tire size comparison calculator and make sure the new
size is available in an adequate load index.
Frequently asked questions
What does the load index on a tire mean?
It is a standardized code for the maximum weight one tire can carry at its rated pressure. For example, load index 94 means 1,477 lb (670 kg) per tire. Multiply by four for the theoretical vehicle maximum, though the placard rating of your vehicle is what legally matters.
Can I fit tires with a lower load index than stock?
No. The replacement tire must meet or exceed the load index on your vehicle placard or original equipment. A lower index means the tire can overheat and fail under loads your vehicle is designed to carry, and it can void insurance coverage in some jurisdictions.
What is the difference between SL and XL tires?
XL (extra load) tires have a stronger construction that carries more weight at higher inflation pressure than a standard load (SL) tire of the same size. If your car came with XL tires, replace them with XL: an SL tire of the same size has a lower effective load capacity.
What do the load index numbers on dually or LT tires mean?
LT tires often show two indexes, like 121/118: the first applies in single fitment, the second when twinned on a dual rear wheel axle. Duals carry slightly less per tire because the inner tire runs hotter.