Tire Size Calculator

[NorCal 4x4]

I found three useful tire size calculators.

The first is for finding the diameter or any combination of: section width, aspect ratio, rim diameter, and tire diameter, (using three of the four as inputs)! This link also has calculators/tables for tire size affect on gear ratio, fuel economy, speedometer, etc.
1) 4LO Metric Tire Conversion Then select "metric tire conversion" tab at the top of the page. Some of the calculators are partially hidden on my screen, so just right click on the tab for the calculator you want to use and "Open in New Window".

The second one shows a diagram comparing your current tire size to your proposed tire size. Shows either in height, or the width:
2) Visualize Change

The third shows the percentage of odometer and speedometer change for tires other than OE:
Example:
Assuming starting with an Off-Road or Big Tow Pkg. then switching to 305/75R/17 tires, (after lift, and not accounting for any other factors than tire size):
Data
OE Tire Width (mm) --- 285 ------ New Tire Width (mm) --- 305
OE Aspect Ratio --------- 70 ------ New Aspect Ratio --------- 75
OE Rim Diameter (in) --- 17 ------ New Rim Diameter (in) --- 17
Sample Speed ----------- 60 ------ Sample Travel Distance: --- 1
OE Gear Ratio -------- 3.357
RESULTS
OE Tire Diameter (in) ------- 32.7 - New Tire Diameter (in) ------- 35
Tire Dia. Difference (%) ---- 7.04 -- Difference Direction: -- Larger than OE
Original Odometer Reading ---- 1 -- New Odometer Reading: -- 0.934
Original Speed Reading ------- 60 -- New Speed Reading: ------ 56.05
Instrumentation Error (%) -- 7.04 - Error Direction: -------- Less than actual
Effective Gear Ratio ------------------------------------------------ 3.136

Here's the link. Remember to change the OE Gear ratio, if you don't have Off-Road or Big Tow.
3) Tire Size Calculator