[dyogim]

Metric To Standard: 285/75/16--width in millimeters, aspect ratio, rim diameter in inches

25.4mm = 1"

1. 285mm/25.4mm = 11.22"(width of tire)
2. 75%(0.75) *11.22" = 8.42"(sidewall height)
3. (8.42" x 2) +16" = 32.83" (overall height of tire)

The closest size in inches would be 33x11R16.

Standard to Metric: 35x12.5R16--height, width and rim diameter all in inches

1. 12.5*25.4 = 317.5mm(width of tire)
2. (35-16)/2 = 9.5 (sidewall height)
3. 9.5/12.5 = 0.76 (76%) (aspect ratio)

Which corresponds closest to a 315/75/16.

How big of a difference does it make?
Stock tires are 245/70/16. The circumference of this tire is 92.69" and it takes 683.57 revolutions to go 1 mile. I have 285/75/16's. So, we'll use this as an example. For a 285/75/16 the circumference is 103.14" and it takes 614.31 revolutions to go 1 mile. The stock tire is what the cpu is reading.

1 mile = 63360"

1. Multiply the numbers of the stock revoution with the circumference of the bigger tire. In my case,
683.57 x 103.14" = 70503.4098"
2. 70503.4098" / 63360" = 1.11 miles

A difference of .11 miles from stock size tires to bigger tires over 1 mile means add 11% to the mileage read on the odometer and the speed on the speedometer.

Quick Reference
Assuming stock 3.91 gears and 265/70/16 (30.6") tires, add the following percentage to the gauges for a correct reading with larger tires and stock gears:

265/70/16 (30.6") Normal speedometer reading if so equipped. Not sure on stock 245/70/16 trucks.
265/75/16 (31.64") add 3%
285/75/16 (32.83") add 7%
315/75/16 (34.6") add 13%


The quick route to the percentage gain is to divide the larger diameter by the smaller diameter, subtract 1 and multiple by 100 to get the percentage to add to the speedometer and odometer.

Tire Size and Gear Ratios
For the greatest all-around efficiency, use the gear ratio closest to the "ideal" ratio and on the high side numerically. For offroad oriented builds, climbing while towing or hauling, or primarily in-town driving, use the higher of two ratios if listed.

265/70/16 Tire, 3.91 Gearset = Stock Reference
(Tire Size, Ideal Ratio => Closest Available Ratio)
285/75/16, 4.19 => 4.10 or 4.30 (Standard 33" tire)
255/85/16, 4.22 => 4.30 (Narrow 33, slightly taller than the other two)
305/70/16, 4.19 => 4.30 (Wide 33, most rotating mass of common 33s)
315/75/16, 4.42 => 4.56 or 4.88 (Standard 35x12.5)
315/80/17, 4.70 => 4.88 ("Standard" 37x12.5R17)
365/75/17, 4.92 => 5.xx - various non-Toyota, or 5.29 (38.5x14.5, SAS prob. req!)