tire diameter ? need help understanding
Thread Starter
TECH Fanatic
Joined: Oct 2002
Posts: 1,068
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From: Seattle,Wa.
tire diameter ? need help understanding Chuck @ discount tire informed me that my Nitto 555, 245 50R 16" tire diameter is 25.65.
The Mickey Thompson & Bfg force 2s tire diameter for 255 50R 16" is 26.1
I'm talking ET street radials.
I thought because all three tires were 50R that the diameter would be the same.
If I ran the Mickey Thompsons on my car would I have to make changes for my shift points, or for my speedometer as well because of the larger diameter size tire.Maybe it's not enough of a difference to matter.
Can anyone explain why the tire reading of 50R is the same on all three tires but the diameter is not the same.What am I missing here.I need to be edumucated.
Thanks in advance.
The Mickey Thompson & Bfg force 2s tire diameter for 255 50R 16" is 26.1
I'm talking ET street radials.
I thought because all three tires were 50R that the diameter would be the same.
If I ran the Mickey Thompsons on my car would I have to make changes for my shift points, or for my speedometer as well because of the larger diameter size tire.Maybe it's not enough of a difference to matter.
Can anyone explain why the tire reading of 50R is the same on all three tires but the diameter is not the same.What am I missing here.I need to be edumucated.
Thanks in advance.
Staging Lane
Joined: Sep 2004
Posts: 93
Likes: 0
From: Charlotte, NC
to elaborate...the "50" means 50% of 245 or 255, so obviously 50% of 255 is going to taller that that of a 245. Now, if your going to size up from the stock 245-50-16, if i'm not mistaken, the only option that will fit is a 255-45-16. it should be about as tall or a tad bit taller (diameter in this case), but really close either way. I didn't even know there was a 255-50. also, the sidwall is going to make the tire look "thicker" if you know what i mean. and as far as the speedometer, you're only talking about 1 mph difference MAYBE, so i wouldn't bother changing shift points.
TECH Apprentice
Joined: Dec 2004
Posts: 349
Likes: 1
From: Scottsdale, AZ
A 245/45R16 is only 24.68", while a 245/50R16 is 25.65". A 255/50R16 will be 26.00", or only 0.35" taller than the 245/50. I used 255/50R16 tires on my car for a while, and didn't have any problems. FYI for a 245/50R16 tire, the 245 is the section width (widest part of the tire, NOT the tread patch) in millimeters. The 50 is the aspect ratio (% of section width) and 16 is the rim diameter. So, you can calculate any tire size with the following formula:
(Section width x Aspect ratio) x 2
-------------------------------- + rim diameter
25.4
Just remember to put the aspect ratio in as a decimal instead of a percentage (50%=.50, 45%=.45, etc.). This gives you the overall diameter of any tire based on it's size. As an example: 245/45R16 - section width (245mm) times the aspect ratio as a decimal (45%, or .45) times 2 (tire sidewall on both sides of the wheel) 245mm x .45 x 2 = 220.5 mm. Divide by 25.4 (to convert millimeters to inches) 220.5 /25.4 = 8.68 inches. Add rim diameter 8.68 in + 16 in = 24.68 inches. Simple, huh?
(Section width x Aspect ratio) x 2
-------------------------------- + rim diameter
25.4
Just remember to put the aspect ratio in as a decimal instead of a percentage (50%=.50, 45%=.45, etc.). This gives you the overall diameter of any tire based on it's size. As an example: 245/45R16 - section width (245mm) times the aspect ratio as a decimal (45%, or .45) times 2 (tire sidewall on both sides of the wheel) 245mm x .45 x 2 = 220.5 mm. Divide by 25.4 (to convert millimeters to inches) 220.5 /25.4 = 8.68 inches. Add rim diameter 8.68 in + 16 in = 24.68 inches. Simple, huh?
