First off, what measurement is used for tires?

My chip has a thing where you can enter the stock tire size and new tire size and it'll tell you how fast you are actually going, but it asks you to enter the sizes in inches.

I took a guess at the original measurements being in centimeters so I entered the stock tire size (245) as 96.45" and put the new tire size (315) as 124", but what my chip is telling me doesn't seem right. When my truck speedometer says 40mph, the chip says 52mph.

So 1) am I doing the conversions right? and 2) are chips credible enough to be used accurately to gauge speed? (It's an Edge juice w/ attitude chip on an '02)

Just wondering

 

That a tape measure and measure the tire on level ground from the bottom to the top of the tire.If you want to get very accurte use a level.It should be in inches.

If the tires are 31 1/2 inches,it will be 31.50, or if they are 31 1/4 inches it will be 31.25.Does this help?

 

Yeah I guess that might work too, but I also need to get the measurement from a stock tire

 

What were the stock tire size on the truck?

 

The sticker on the door says 245/75/r16

 

the 245 and 315 are measurements of the sidewall in mm IIRC.

 

That is 30.5 inches tall.But it will be less when it is on the truck and weight is on the tire.

 

so how can i convert that into inches?

 

Convert mm to inches - Conversion of Measurement Units

 

Alright I looked at that and it gave me the same numbers i had, just one decimal to the left, but I can put it into the chip configurations in the morning and see if it makes any difference. thanks guys