Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - View Single Post - 2 experts debate tire revolutions/mile (vs. measured circumference)

CapriRacer · 04-18-2015, 07:39 AM

What prompted this thread is a PM I sent Cosmick. I suggested we do an off-line discussion to resolve the issue. I don't think embarrassing someone in public is the right way to get them on the right page, but it's obvious that isn't the way this is going to go down.

I'll quote from some of the PM I sent:

"......Where am I coming from? I'm a retired tire engineer - 40 some years. From the start of my career, I have been aware that this whole issue is problematic for the layperson - that the rolling diameter can NOT be measured directly.

For a while, I was a development engineer and would be the guy who published the dimensions of tires, including revs per mile. We had a formula we used and the result was about a 3% difference compared to the free standing (that is unloaded) diameter. But at the same time, we verified these dimensions where we could to assure their accuracy - and that included revs per mile. If I remember correctly we had a counter on a test wheel that was read while some other test was going on.

Plus, I was, for a time, a liaison to Ford, and we would pass dimensional information to Ford, so they could check for clearances and speedometer accuracy (which needed to be within 3% by regulation). They never mentioned any errors in the data we reported.

So I am extremely confident in the 3% value - keeping in mind it's an approximation and varies according to load/inflation pressure/speed.

What I would propose is that we both measure tires. Unfortunately the only vehicle I have on a hard surface is a 2006 Mercury Milan, so I will only have 2 data points - front and rear.

I suggest you do the same. One word of caution: Be careful measuring the back tires on an unloaded pickup - especially a 250 or 350 using Ford's terms (2500 and 3500 using Chevy's terms). Those rear tires can be so slightly loaded that the rolling diameter might be less than a 3% difference...... "

Well, there you have it. I've proposed a verification and will report on it when I have completed it. I'm going to suggest that others do this as well.

I appreciate the effort Cosmick put into obtaining his answer, but I'll submit that the tire manufacturers thoroughly disagree and that fact alone should be enough to cause hesitation on contradicting them.

I'll be back later with a description of my methodology and details about the results.

04-18-2015, 07:39 AM	#8 (permalink)
CapriRacer Tire Geek Join Date: May 2008 Location: Let's just say I'm in the US Posts: 796 Thanks: 4 Thanked 393 Times in 240 Posts	What prompted this thread is a PM I sent Cosmick. I suggested we do an off-line discussion to resolve the issue. I don't think embarrassing someone in public is the right way to get them on the right page, but it's obvious that isn't the way this is going to go down. I'll quote from some of the PM I sent: "......Where am I coming from? I'm a retired tire engineer - 40 some years. From the start of my career, I have been aware that this whole issue is problematic for the layperson - that the rolling diameter can NOT be measured directly. For a while, I was a development engineer and would be the guy who published the dimensions of tires, including revs per mile. We had a formula we used and the result was about a 3% difference compared to the free standing (that is unloaded) diameter. But at the same time, we verified these dimensions where we could to assure their accuracy - and that included revs per mile. If I remember correctly we had a counter on a test wheel that was read while some other test was going on. Plus, I was, for a time, a liaison to Ford, and we would pass dimensional information to Ford, so they could check for clearances and speedometer accuracy (which needed to be within 3% by regulation). They never mentioned any errors in the data we reported. So I am extremely confident in the 3% value - keeping in mind it's an approximation and varies according to load/inflation pressure/speed. What I would propose is that we both measure tires. Unfortunately the only vehicle I have on a hard surface is a 2006 Mercury Milan, so I will only have 2 data points - front and rear. I suggest you do the same. One word of caution: Be careful measuring the back tires on an unloaded pickup - especially a 250 or 350 using Ford's terms (2500 and 3500 using Chevy's terms). Those rear tires can be so slightly loaded that the rolling diameter might be less than a 3% difference...... " Well, there you have it. I've proposed a verification and will report on it when I have completed it. I'm going to suggest that others do this as well. I appreciate the effort Cosmick put into obtaining his answer, but I'll submit that the tire manufacturers thoroughly disagree and that fact alone should be enough to cause hesitation on contradicting them. I'll be back later with a description of my methodology and details about the results. __________________ CapriRacer Visit my website: www.BarrysTireTech.com New Content every month!