got some new tires (285/55/18) and the calculator says 30.3" but when i actually measure them their right at 29"? i've done this with multiple tires, are they counting the tire expanding or something when its turning?

 

Are they toyo's? i hear they are pretty far off. and i believe the advertized height is unloaded height.

 

i think i remember somebody saying the correct way to measure them is to draw a line on the tire, pull the truck foward and get the line where it was.. take a length measurement of the length traveled and divide by 1.33?
I better start lookin

 

Tire manufacturers aren't regulated in tire sizing. They can make whatever they want, basically. Usually they are close, but never the same, especially when you compare rolling circumference. This is why it is recommended to run matching sets of tires on your vehicles.

 

Depends on how you are measuring it, which you failed to state. The best way to measure the diameter is like nonnie said - get the circumference and calculate the diameter.

 

Circumference=PI*Diameter
so
Diameter=Circumference/PI

PI=3.14

 

If they're Toyo's, then they will measure about an inch smaller then the calcs say they should be, at least the 2 set's I've gotten do...

 

they are cooper zeon. does anyone know the exact way to measure them using the cicumference?

 

See SilverDog's post above^^^

 

It is mentioned in post #3