tire calculator In the tech section

[rooster]

I have been screwing around with thing enough! http://www.roddingroundtable.com/tech/articles/calc2.html

What is a
aspect ratio?

[enjenjo]

The ratio between the height, and width of the tire. It is generally the second number on the sidewall,as in 195 60, the aspect ratio is 60,  255 70,  the aspect ratio is 70.

By the way, the first number is the over all width of the tires in millimeters. So on a 195 60, the tire is 195 mm wide, and 60 percent of 195 mm high from the bead to the tread.

[34ford]

Denny,
Here's one that might be a little easier to understand and work. http://www.project33.com/tiresize.cfm

bob

[Bob Paulin]

While the calculators mentioned above are valuable tools for determining  speed in as it relates to RPM for a given situation, I don't want to get out the computer every time I deal with tire size changes.

Carnuts.com has a simple chart to help you maintain the same tire diameter while changing aspect ratio....for example, if you would like to replace a 215-75R-15 with a 60 series tire.
 
Under the "75" series heading, the chart shows that the 215-75R-15 tire is approximately 27.7-inches in diameter.

Moving over to the "60" series section, we find that the 265-60R-15 is approximately 27.5 inches in diameter while the 275-60R-15 is 28 inches in diameter - differences of .2 and .3 inches respectively.

The "75" series tire runs 728 revolutions per mile while the "60" series run 733 and revs per mile 720 respectively...differences of .0068 more revs per mile and approximately one percent fewer - approximately .5 to .6 mph over or under at 65 mph.

Maine State Police will allow us at least 5 mph at 65 - which is a 7.6 percent allowance, so the small percentage that the speedo might be off is really insignificant to the health and well-being of your driving privilege or your valuable "points" (I feel like a roller derby announce - "Five more valuable Warrior points").

My printed out chart resides safely in "Bob's Big Book of Stuff".


Bob

[nitrofc]

http://www.1010tires.com/tiresizecalculator.asp