Tire Height Information

This formula is a quick way to get the tire diameter of those metric tires that are common on just about everything stock. For example a LT265/75R16 would be around 31.6 inches tall and 10 inches wide. "LT" means Light Truck and "P" means Passenger tire. The bigger number (on the left) is the Section Width. The number to the right of the slash ("/") is the Aspect Ratio (percent of width). The "R" means Radial tire and the last number, far right, is the rim diameter in inches.

Section Width
Aspect Ratio
Rim Diameter
Tire Diameter (inches)
Speedometer Information

The following formula will work for any GM, Ford or Chrysler vehicle.Drive gear refers to the gear that is on the transmission output shaft. The driven gear is located in a removable housing usually in the tailhousing of the transmission.

Numberof Drive Teeth
Axle Ratio
Tire Diameter(inches)
Number of Driven Teeth


Numberof Driven Teeth
Axle Ratio
Tire Diameter(inches)
Number of Drive Teeth
Caculate Your RPM
MPH
Tire Diameter ( in inches )
Axel Ratio to 1
Transmission High Gear Ratio to 1
RPM

Caculate Your MPH
RPM
Tire Diameter ( in inches )
Axel Ratio to 1
Transmission High Gear Ratio to 1
MPH

Follow the speed measuring instructions below:
  1. Travel at a constant rate of speed on an open highway and have someone record the number of seconds it takes to travel exactly one measured mile.
  2. On the chart, locate the number of seconds it took your vehicle to travel one mile. The speed indicated next to the number of seconds is the true average road speed of the vehicle.
  3. Repeat the measurement several times and average the results. Recalibration of your speedometer is recommended if significant inaccuracy is noted.
Seconds Per Mile Road Speed
50 72
51 71
52 69
53 68
54 67
55 65
56 64
57 63
58 62
59 61
60 60
61 59
62 58
63 57
64 56
65 55.4
66 54.5
67 54
68 53
69 42