Need Help w/ Understanding Tire Sizes [Archive] - CustomTacos.com Forum

Need Help w/ Understanding Tire Sizes

butters
02-25-2004, 12:49 PM
I know the last # means the size of the rim, but what do the first two numbers mean?

For example, what is the difference between a 265/70R16 vs. a 265/75R16?

**DONOTDELETE**
02-25-2004, 12:54 PM
the second number is the width, and the first number is the circumfrence, or height, something like that. a 265 is a 31" tire, and a 275 is a 32", and a 285 is a 33".

99TRDTaco
02-25-2004, 01:11 PM
The first number is the section width in mm (not the tread width) the section width is the widest part from sidewall to sidewall a tire with a section width of 10.5 inches (265mm) will be anywhere from 9 to 9.5 inches at most. The second number is a ratio of the section width to sidewall height. In other words if you have a 265/75/16 the section width is 265mm or approximately 10.5 inches the sidewall height is 75% of that width and 16 is the rim diameter. Any more questions let me know and I will try to clarify more.

99WhiteRice
02-25-2004, 01:18 PM
actually, the first # is the distance in mm from the bead of the tire to the center and the second # is the percentage of that first # that is the side wall.

for example 265/70/16 (70% of 265 is 185.5) wich means the side wall is 185.5 mm tall and the 79.5mm that is left from 265 (265-185.5=79.5) is 1/2 of the contact patch/surface/width so total contact area would be 159mm (79.5x2=159)
(it ends up being a 30.6 inch tire)

my 215/35/18's have a side wall of 75.3mm and are 279.4mm wide and 23.9 inches tall
hope this shows you how it works

99TRDTaco
02-25-2004, 01:23 PM
my 215/35/18's have a side wall of 75.3mm and are 279.4mm wide and 23.9 inches tall



This would mean that your tire is over 10 inches wide (279.4mm/25.4) are you sure thats right? http://www.CustomTacos.com/ubbthreads/images/graemlins/shrug.gif

99TRDTaco
02-25-2004, 01:29 PM
This link was posted in the 2wd tire and wheel section.
http://www.tirerack.com/tires/tiretech/general/size.htm

NAD
02-25-2004, 01:32 PM
The first number is the section width in mm (not the tread width) the section width is the widest part from sidewall to sidewall a tire with a section width of 10.5 inches (265mm) will be anywhere from 9 to 9.5 inches at most. The second number is a ratio of the section width to sidewall height. In other words if you have a 265/75/16 the section width is 265mm or approximately 10.5 inches the sidewall height is 75% of that width and 16 is the rim diameter. Any more questions let me know and I will try to clarify more.

This is correct.

To convert to inches, use this formula:
((Width * Aspect Ratio * 2) / 2550) + Wheel diameter = height in inches

For example, my 215/75R15 tires would be as follows:
((215 * 75 * 2) / 2550) + 15 = 27.64" tall

S-Dad
02-25-2004, 01:34 PM
Actually the first number is you width in millimeters, divide by 25.4 for inches. The second number is you aspect ratio or percent factor of your side wall height to your tire width. The 3rd number is your wheel diameter. For a 265/70/16 tire your width is 10.43 inches(265/25.4) and your side wall height is 7.3 inches(10.43x70%). Your total diameter is 30.6 inches or twice your side wall height plus wheel diameter.

Looks like I was a little late in posting...gotta hit that send button faster.

99TRDTaco
02-25-2004, 01:39 PM
Actually the first number is you width in millimeters, divide by 25.4 for inches. The second number is you aspect ratio or percent factor of your side wall height to your tire width. The 3rd number is your wheel diameter. For a 265/70/16 tire your width is 10.43 inches(265/25.4) and your side wall height is 7.3 inches(10.43x70%). Your total diameter is 30.6 inches or twice your side wall height plus wheel diameter.

Looks like I was a little late in posting...gotta hit that send button faster.


I thought that I had heard that somewhere before http://www.CustomTacos.com/ubbthreads/images/graemlins/lol.gif http://www.CustomTacos.com/ubbthreads/images/graemlins/lol.gif http://www.CustomTacos.com/ubbthreads/images/graemlins/lol.gif http://www.CustomTacos.com/ubbthreads/images/graemlins/lol.gif

99WhiteRice
02-25-2004, 01:39 PM
hummm. i don't know where i went wrong, the math seems right but i just went and measured my tires and they're a little over 8"s wide wich is about 209mm wide, but thats not what the math says that weird http://www.CustomTacos.com/ubbthreads/images/graemlins/shrug.gif

Boshak 4x4
02-25-2004, 03:12 PM
here's a calculator if that helps?

http://www.4wheelerssupply.com/Tech/tire_calculator.php

butters
02-26-2004, 06:14 AM
Cool calc.

I guess a 265/75R16 is a 32" tire where a 265/70R16 is a 31" tire

I have the stock 265/70R16's on my truck now. Would I be able to go up 1 inch to a 32" tire without any lift?

Boshak 4x4
02-26-2004, 10:36 AM
i think andy [inacoma] is running no lift in the rear with 32's. i'm not sure about keeping the front stock with 32's. i'm gonna guess you'll rub pretty hard considering i've heard people with 2" lifts rub at full turn on bumps.

also. if you haven't seen this http://www.customtacos.com/tech/index.php?page=index_v2&id=137&c=10 if you click on tire size guide or scroll all the way to the bottom, it tells you what fits pretty well with different lifts. 0-2" recommends a 31" tire for no rubbing. 3" is recommended for a 32" tire.

99TRDTaco
02-26-2004, 11:36 AM
As long as you keep the stock wheels you will have very minimal rubbing. Check out TTORA there were a couple of long posts about this subject not to long ago.

trdan01
02-26-2004, 12:02 PM
Hey butters, I did a little write up a while back because you, me and a lot of other people have had the same question:

http://homepage.mac.com/danny/tacoma/tires/index.html

hope this helps.

foofighter
02-27-2004, 01:17 AM
this has got to be the best post i've read thus far! I'm new to the truck thing and have always been confused when people would talk 31" this or 32" that now i know thanks!

quick question though regarding tire pressure...what's ideal for normal city driving?

**DONOTDELETE**
02-27-2004, 08:05 AM
quick question though regarding tire pressure...what's ideal for normal city driving?


Read the sidewall of your tire, and it'll specify a range of acceptable pressure -- fill to the top of that range.

trdan01
02-27-2004, 11:20 AM
Actually not all tires list this on the sidewall. My BFG AT KO's don't list a range, they just list a maximum PSI (under max load) which is like 60 or 65, way too high for normal driving conditions. I run mine at 32 up front and 33 in back.