|Home||Forums||Reader's Rigs||Tech Library||Trail Stories||FSJ-List|
Tires, Rims, & Gearing
Contributed By: Joe Schaefer
Measured from stud to stud:
Section width: width of the tire between sidewalls NOT the tread width. Usually measured in mm and the first number on a metric tire. Such as the "235" on a 235/75R-15 tire.
Aspect Ratio: Relationship between section height/width.
Tire diameter of a metric tire can be figured using:
tire dia. = 2 x section width x aspect ratio + wheel size 25.4 100
tire diameter = mph x gear ratio x 336 rpm
gear ratio = rpm x tire diameter mph x 336
mph = rpm x tire diameter
rpm = mph x gear ratio x 336
NOTE: For rpm's other than hwy speeds you need to figure in trans gear ratio and x-case ratio. For mph at hwy speeds use 5th gear OD ratio ie. 072:1 etc.
Actual speed: When changed to new tires and you need the actual speed withhout changing speedo drives. Bigger tires cause speedo to read slower than true speed.
actual speed = new tire dia. x indicated speed old tire dia.
Example: Old tires 28 inch, new tires 35 inch and speedo reading 60 mph
35 x 60 mph = 75 mph true speed
To figure what indicated speed to run to achieve actual speed use:
indicated speed = old tire dia. x actual speed new tire dia.
Actual new gear ratio with new tires:
new gear ratio = old tire dia. x old gear ratio new tire dia.
Gears to install to restore original gear ratio after new tires:
new ratio to install = new tire dia. x original gear ratio old tire dia.
IFSJA.org hosting provided by Golden Computer Service.
© 2000-2006 IFSJA Site Staff. Jeep® is a registerd trademark of DaimlerChrysler.