You can figure the tire diameter using the size of the tire. The first number is the width, the second is the aspect ratio. For instance, a 275/60/15, you mulitiply 275 x .60 to get the tire's sidewall height, then mulitiply times two, divide by 25.4 (to convert to inches, the width is in millimeters) and add 15 to get the diameter. To get the circumference, multiply the diameter times Pi which is 3.1416.---------- 275 x .60 = 165-----165 x 2 = 330------330/25.4 = 12.99 + 15 = 27.99---- 27.99 x 3.1416 = 87.94. Another way to find the rpms at 60 mph is to divde the circumference into 63,360 (inches per mile) 63,360/87.94 = 720.5, multiply this times the rear gear ratio to get rpms @60mph 720.5 x 3.55 = 2558 (1 to 1 final drive) for overdrive multiply this times the OD ratio 2558 x .63 = 1611
5.0L, T5Z and 3.40TL and 1700RPM at 60MPH. Great combo for pickup and freeway cruising as it's like having a 3.58:1 rear with a toploader in 1st, but a 2.14:1 rear gear with a toploader in the T5z's 5th.
Daniel