what's the best way to measure true tire size? toyo specs mine at 37.1", the method from the hypertech programmer manual gives me 35.3". i split the difference, but don't know if that's right. anybody know a better way?

I'm only half joking here: Air up you tires the way you normally do; put a thin wet paint strip on the top of one tire tread perpendicular to the rotational direction of the tire; drive forward for about 15 feet and measure the distance on the pavement between the two paint marks; divide that number by Pi - 3.14 (3.14159265358979323846...). That's the effective diameter of your tire during act of driving. It's a little bit of a pain, but it's probably the most accurate way to do it.

instead of a paint strip, i used a chalk mark, and did the math. just wanted to know if anyone had other ways to do it.

I assume that the chalk/paint method is the most accurate because it measures the actual radial length of your tire (diameter x Pi) when actually driving it. When measuring the diameter directly, you might get pretty close, but you have to guess a little about where to measure and whether or how much the top of the tire will compress under load.

Another way to do it is to zero a trip odometer as you pass a mile marker on a fairly-striaght highway; drive, say to 20 miles; as you pass the 20th mile-marker; see what your odometer reads. If, for example, your odometer read 19.5 mi, your speedo and odo are reading 2.5% too low, meaning that your tires are 2.5% larger than the size for which the DIC is programmed.

BTW, if your state/local highway project workers are stoned, the results might vary. The above method also won't work well if you have to stop off and get beer along the way. Basically, the more miles you drive, the more accurate your results should be - but I figure 20 should be fine.

Well, the memories of listening to your geometry teacher probably wouldn't be as worthy as those of the drinking games and girl-chasing(unless the teacher had really big tatas and, then, you wouldn't really be listening as much as focusing).

Yes, and by measuring the circ when driven, it is the circumpherence under load.

thanks, i feel like i should be wearing my helmet, waiting at the door for the bus to pick me up . . . .

Re: tire size
 

I just talked to a Hypertech tech on the phone about that today. He said he just measures the tire from the top to the ground or take the radius and multiply by 2. Either way is accurate.

Re: tire size
 

:giggling:

accurate wouldn't be accurate

Do you use the radius from the hub to the bottom, top, sides?:giggling:

Measuring the height of the tire while on the vehicle should provide an incorrect number, albeit a who-cares kinda incorrect.

When determining the speed of the vehicle and mileage, it obviously measures how much the circumference of the tire travels. Wouldn't matter if the tire was oblong running on tracks, the circumference is what matters, not the diameter or height.

Since it actually is NOT completely oblong but the bottom is flattened due to vehicle load, it's still the same association. Lift the tire, take the circumference and divide by Pi and that should provide the most accurate number to use for programming when changing tire sizes. Especially since tire companies list their tire sizes as they sit off the vehicle, not under load.

Re: tire size
 

I hear ya shhrtbs. Think I'm gona have me a drink while I wait. :D