Let's talk about tire diameter.
03-18-2009, 03:34 PM
#12
Cut & Paste Expert
Join Date: May 2007
Location: NW Arkansas
Posts: 13,322
:115: The answer is in the weight of the car. :115:
When you put the weight of the car on the tires, two things happen: the sidewall compresses, and the tread flattens out. Flattening out the rubber tread against the ground effectively decreases the overall circumference of the tire. This gives you more revolutions per mile than using the simple calculation above.
Think "Silly putty on a news paper." (An over-simplification, I know, but you get the point.)
When you put the weight of the car on the tires, two things happen: the sidewall compresses, and the tread flattens out. Flattening out the rubber tread against the ground effectively decreases the overall circumference of the tire. This gives you more revolutions per mile than using the simple calculation above.
Think "Silly putty on a news paper." (An over-simplification, I know, but you get the point.)
If the Revs/Mile changes that much to make a difference,
your tires would last no time at all. The tread cant deform
that much when it conforms to the road surface in the
form of a contact patch to change the revs/mile enough
to notice any amount of different speedo readings.
So, now we realize a need to use a different calculation to get the value you need to properly correct the speedometer. This leaves you with two choices.
I use the second method, because while it's hard to get a measurement that's 100% accurate even to 1/10th of an inch, you can get pretty close with a simple tape measure. It's also a lot less work than marking the ground/tire and rolling the car around.
The next time you think about changing tires, hopefully now you know how to figure out the proper diameter and revolutions per mile you have to enter into your programmer.
- Mark the tire and the ground. Then roll the car forward one complete revolution, measuring the distance between the marks.
- Take the distance from the center-line of the wheel down to the surface and multiply it by 6.283.
I use the second method, because while it's hard to get a measurement that's 100% accurate even to 1/10th of an inch, you can get pretty close with a simple tape measure. It's also a lot less work than marking the ground/tire and rolling the car around.
The next time you think about changing tires, hopefully now you know how to figure out the proper diameter and revolutions per mile you have to enter into your programmer.
with the weight of the vehicle on the tire and measure the
overall diameter. You will find that it very closely matches
the diameter when the tire is jacked up and unloaded from
the vehicle weight.
Last edited by 157dB; 03-18-2009 at 03:40 PM.
03-18-2009, 05:31 PM
#14
6th Gear Member
Join Date: Sep 2006
Location: Gainesville, FL
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03-18-2009, 07:47 PM
#15
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Thread Starter
Join Date: Dec 2007
Location: Coral Springs, Florida ~ 33076
Posts: 608
I would have to agree to disagree.
If the Revs/Mile changes that much to make a difference,
your tires would last no time at all. The tread cant deform
that much when it conforms to the road surface in the
form of a contact patch to change the revs/mile enough
to notice any amount of different speedo readings.
If the Revs/Mile changes that much to make a difference,
your tires would last no time at all. The tread cant deform
that much when it conforms to the road surface in the
form of a contact patch to change the revs/mile enough
to notice any amount of different speedo readings.
I think I went through this pretty thoroughly, but here's my spreadsheet. If I'm wrong, please help me figure this out so I can fix the write up.
You're talking about a difference of up to 3 inches in diameter between loaded and unloaded circumference.
The pure math of the whole thing gives you one number, but the actual and physical measuring gives you significantly more revolutions per mile.
Lucy, you've got some splainin' to do.
Last edited by Big Top Gt; 03-18-2009 at 07:49 PM.
03-18-2009, 07:48 PM
#16
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Thread Starter
Join Date: Dec 2007
Location: Coral Springs, Florida ~ 33076
Posts: 608
The old fashioned way to check a speedo is to drive 1 mile at 60 mph. If it takes 60 seconds your speedometer is correct.
I've been just using the special speedometer check 5 mile sections the interstates around here have and comparing with the odometer. I'll do it the old fashioned way one of these days just to compare.
I've been just using the special speedometer check 5 mile sections the interstates around here have and comparing with the odometer. I'll do it the old fashioned way one of these days just to compare.
How can you calculate the Revolutions Per Mile correction in your tuner using that method, though?
This information is basically wrapped around my need to fix the tire RPM's in my X3.
03-18-2009, 09:25 PM
#18
Join Date: Feb 2008
Location:
Posts: 8
Best Tire Calc That I've Found
When changing from the stock 17 wheels and 235/55/17 tires to Steeda 18 X 9.5's with 275/40/18's I noticed that the new tire was actually shorter in height due to smaller sidewall. This tire calc was the best that I found showing revs per mile, tire height circum. etc.
http://net-comber.com/tirecalc.html
http://net-comber.com/tirecalc.html
03-19-2009, 11:25 AM
#19
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Join Date: Dec 2007
Location: Coral Springs, Florida ~ 33076
Posts: 608
Now that I think about it a little more, I bet a little hand held GPS would be helpful in the matter, too. Can't you get some that have a speedo built in based on the satellite?
03-19-2009, 11:32 AM
#20
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Join Date: Dec 2007
Location: Coral Springs, Florida ~ 33076
Posts: 608
When changing from the stock 17 wheels and 235/55/17 tires to Steeda 18 X 9.5's with 275/40/18's I noticed that the new tire was actually shorter in height due to smaller sidewall. This tire calc was the best that I found showing revs per mile, tire height circum. etc.
http://net-comber.com/tirecalc.html
http://net-comber.com/tirecalc.html
Plug in the tire sizes used in my example and compare the calculated revs per mile compared to what the manufacturer gives you.
The Pirelli, for example, is short by roughly 25 revs per mile, whereas the ET Street is off by as much as 15. It's closer to some, but the math still isn't spot on.