Math Question
Seasoned HDF Member
Joined: Apr 2008
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From: Sandpoint, Idaho
Road Master
Joined: Aug 2008
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From: Slidell, Louisiana
What formula do you use?
Club Member
Joined: Jun 2007
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From: Bedford UK
The short answer to your original question is that the larger tyre is less than 1% bigger, so the difference is not big enough to worry about. Both tyres will have more than 1/2" of tread, on diameter, so the difference in diameter is much less than the tread depth. At some time during its life a 140/90 will be the same diameter as a 140/85.
Road Captain
Joined: Nov 2011
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From: Illinois Corn Desert
85 divided by 90 equal .9444. 70 mph times .9444 equals 66.11 mph actual speed. Speedo would still read 70 mph. To be going 70 mph your speedo would need to read 73.962 or about 74 miles per hour then your actual speed would be 70 mph.
Road Captain
Joined: Nov 2011
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From: Illinois Corn Desert
Correct. I did the math using the numbers provided. Not knowing actual diameter of both tires leaves room for error. Math was more for an explanation of how to do the calculations for one's own use at a later time when the actual variables are known.
Last edited by Mark5; Mar 7, 2013 at 06:54 AM. Reason: spelling
Club Member
Joined: Jun 2007
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From: Bedford UK
So 25.1 divided by 25.6 equal .98. 70mph times .98 equals 68.6mph. What's a mph or so between friends!
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Road Captain
Joined: Nov 2011
Posts: 547
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From: Illinois Corn Desert
I did the math using the numbers provided. Not knowing actual diameter of both tires leaves room for error. Math was more for an explanation of how to do the calculations for one's own use at a later time when the actual variables are known.
Cruiser
Joined: Jan 2013
Posts: 149
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From: Treasure Coast, FL
@J2X ... calculated based on diam of 16" plus 2x'height' of tire. HOWEVER, this is only an approximation, but should be 'close enough' to get a feel for what the section height change is going to do to RPM, and is what I think the OP was after.
As many have pointed out there are several factors that need to be taken into account to get a really accurate answer. The 'specs' tire makers supply are nominal at best so you would really need to get the tires (both) up to normal temp and then measure rolling circumference (we do this on bicycles to get the speedo accurate) by marking a line on the tire, lining that mark up with a mark on the pavement and rolling the bike forward until the mark is again perpendicular with the pavement and measuring the distance.
As you can see this process has lots of room for error and includes the hassle of having to do it for both tires, so the approximation used initially is probably 'good enough' to make a decision on a tire change.
The GPS (while not perfectly accurate for an instantaneous reading) will give you 'real' speed at a given RPM. But I think the OP just wanted an idea of how much of a change would he see.
Cheers
As many have pointed out there are several factors that need to be taken into account to get a really accurate answer. The 'specs' tire makers supply are nominal at best so you would really need to get the tires (both) up to normal temp and then measure rolling circumference (we do this on bicycles to get the speedo accurate) by marking a line on the tire, lining that mark up with a mark on the pavement and rolling the bike forward until the mark is again perpendicular with the pavement and measuring the distance.
As you can see this process has lots of room for error and includes the hassle of having to do it for both tires, so the approximation used initially is probably 'good enough' to make a decision on a tire change.
The GPS (while not perfectly accurate for an instantaneous reading) will give you 'real' speed at a given RPM. But I think the OP just wanted an idea of how much of a change would he see.
Cheers