GarageCalc 
Where to find it and how to read it
Look at the sidewall marking, for example 225/45R17 94V: the 94 is the
load index and the V is the speed rating. Both together form
the service description. Your vehicle placard (door jamb) lists the minimum your car
requires; replacements must meet or exceed it.
The ZR question
Sizes written as 245/40ZR18 confuse people because ZR sits inside the size
instead of after the load index. ZR is a legacy marking for "over 149 mph". The tire's actual
certified limit is the letter in the service description: W (168 mph), Y (186 mph) or (Y)
(above 186). When comparing tires, ignore the ZR and read that final letter.
Practical rules
- Never fit tires rated below your vehicle placard requirement.
- Do not mix speed ratings on the same axle; if mixed across axles, the car's limit is the lowest rating fitted.
- Winter tires one step lower than OE are accepted practice; drive accordingly.
- Speed rating is about sustained capability and heat, not a suggestion that the speed is safe on public roads.
Frequently asked questions
What does the speed rating letter on my tire mean?
It is the maximum speed the tire can sustain safely while carrying its rated load. On a 225/45R17 94V tire, the V means the tire is certified to 149 mph. The rating assumes correct inflation and an undamaged tire.
Can I use a lower speed rating than my car came with?
It is not recommended, and in some places not legal. Speed rating also correlates with handling stiffness and heat capacity, so a lower rating changes how the car drives even at legal speeds. The accepted exception is winter tires, where one step down (for example H instead of V) is common practice; keep speeds correspondingly modest.
What is the ZR marking, like 245/40ZR18?
ZR is an older marking meaning speed capability above 149 mph. Modern tires spell out the actual rating with W, Y or (Y) in the service description, and ZR in the size is kept mostly for tradition. Practically, read the letter after the load index, not the ZR.
Do higher speed ratings wear out faster?
Usually yes, indirectly. Higher-rated tires use softer, grippier compounds and stiffer construction, which tends to mean shorter tread life and a firmer ride. That is the trade you make for the handling and heat tolerance.