Tire size help

[L J]

 

[jjvw]

The first concept to understand is that there are nominal and actual tire sizes. A tire known as a 33" (nominal) is really only ever around 31.5" actual. Refer to it by it's nominal for least amount of confusing. A 285/70R17 is a nominal 32.7" tire which is close enough that it is known as a 33". It will only measure out to around 31.5", which is normal and expected. You are perfectly good to call what you have, "33s"

A good way to get a feel for it is to compare rev/mile numbers. A 285/70R17 will have almost the exact same rev per mile measurement that any 33x will have. They are for all intents and purposes the same height. Just like 265/70R17 is a 32, 265/65R17 is a 31, etc., even though the latter two are known as 31.5 and 30.5, the rounded-up number is close to the flotation or inch size equivalents.

Hub height X2 is all that matters for the speedometer. After that, it is a balancing act between the tire, ride height, shock travel, and body clearance.

 

[machoheadgames]

Hub height X2 is all that matters for the speedometer.

Both work, I prefer rev/mile.

 

[jjvw]

Both work, I prefer rev/mile.

The tire companies do not know my rolling radius and effective tire diameter. I do.

 

[machoheadgames]

The tire companies do not know my rolling radius and effective tire diameter. I do.

The tire does not shrink when you put weight on it, it elongates; therefore the rev/mile is always correct, assuming the manufacturer listed it correctly.

Hub height works if the pressure is correct. If you were to lower the pressure significantly, the hub height would not work. The tire doesn't shrink in circumference just because the hub height changed due to lowering pressure significantly. The hub height doesn't change by enough to matter at normal pressures which is why it works in most cases.

I have checked the rev/mile number across many different brands and models of tires on different vehicles using GPS and back calculating with rpm calculators and all have been 100% accurate using the rev/mile figure....unless I'm checking against a Falken (junk) tire where for some reason Falken doesn't know how to measure and lists it incorrectly. Everything else has been perfectly accurate no matter what vehicle the tire is mounted on and no matter what the pressure is. Good enough for me.

 

[jjvw]

The tire does not shrink when you put weight on it, it elongates; therefore the rev/mile is always correct, assuming the manufacturer listed it correctly.

Hub height works if the pressure is correct. If you were to lower the pressure significantly, the hub height would not work. The tire doesn't shrink in circumference just because the hub height changed due to lowering pressure significantly. The hub height doesn't change by enough to matter at normal pressures which is why it works in most cases.

I have checked the rev/mile number across many different brands and models of tires on different vehicles using GPS and back calculating with rpm calculators and all have been 100% accurate using the rev/mile figure....unless I'm checking against a Falken (junk) tire where for some reason Falken doesn't know how to measure and lists it incorrectly. Everything else has been perfectly accurate no matter what vehicle the tire is mounted on and no matter what the pressure is. Good enough for me.

The effective rolling diameter absolutely changes with tire pressure. We know this, in part, because the tire elongation from decreased pressure increases heat and wear on the tire.

Another way we know changing tire pressure changes the effective tire diameter is because I have been to drag races where drivers have changed tire pressure as a quick way to fine tune the gear ratios.

Good enough is not a refutation of what is measurable and actually occurring.

 

[machoheadgames]

The effective rolling diameter absolutely changes with tire pressure. We know this, in part, because the tire elongation from decreased pressure increases heat and wear on the tire.

Another way we know changing tire pressure changes the effective tire diameter is because I have been to drag races where drivers have changed tire pressure as a quick way to fine tune the gear ratios.

Good enough is not a refutation of what is measurable and actually occurring.

Good enough is down to the exact rpm based on the size of the tire. That is the result I have received based on the advertised rev/mile figure. I can't possibly get it to be more accurate than it is by measuring hub height as opposed to rev/mile.

If you like the hub height method, keep at it. I suggested a method that has worked with 100% accuracy in all cases I have been able to test it.

 

[Rickyd]

Meh, gps vs speedo inaccuracy. Change tooth count accordingly. No need for arguing about tires or looking anything up on the internet. But by all means guys,you do you.

 

[machoheadgames]

Meh, gps vs speedo inaccuracy. Change tooth count accordingly. No need for arguing about tires or looking anything up on the internet. But by all means guys,you do you.

I didn’t even say anything speedo related, I just chimed in that using the rev/mile from manufacturers helps to figure out what tire sizes are equivalent to others. No part of any of that had anything to do with the speedo until I was told only hub height can be used for speedos (which again, I was not talking about).

 

[jjvw]

... until I was told only hub height can be used for speedos (which again, I was not talking about).

Who said only? My assertion is that the tire manufacturers do not know my effective rolling diameter.