Tire width vs section width, vs tire sizes.

[Pentalab]

Just swapped my 275 fronts / 285 rears for my 235 Motivo rain tires I use in the winter.

The 275's appear to have aprx a 9.75" wide tread. The 285's have aprx 10" of tread.

IF I have the story correct, the tire size, like '285' on the prescribed... '10" measuring width' rim is supposed to sorta equal the outer sidewall to inner sidewall dimension...aka section width ?

Both the 275's + 285's are on 10" wide rims. Interesting enough, both have 3 wide rain grooves. (Both are Nitto NT-555's). The rain grooves on the 275's however are slightly narrower vs the 285's. I can't quite get my index finger to lay flat on the 275 grooves, but on the 285's, index finger drops right in.

It appears at 1st glance that the extra 1/4" of total tread width on the 285's.... just got mostly negated by the wider rain grooves.

Whether a tire is mounted on it's min or max recommended width, it appears that the actual tread on the road (with 900+ lbs applied) stays much the same. (and 32 psi used in both extremes).

I noticed some of these different tire brands run wider than other brands. Some are wider, even with the same brand, but different model within the same brand, yet both are... '305', etc.

My 235 Nitto Motivo's have wider treads and bulge more, vs the oem Pirellis, also in a 235-50-18. And both use a 7.5" wide ..'measuring width' for the wheel rim width. And I'm using oem 8" wide rims.

Some of these 305 tires have section widths almost a 1/2" wider than '305'... like 12.4" . 305mm = 12.008".

Are these different brands using.... loose guidelines ?

 

[skwerl]

Yes, different brands use different measuring standards. Best way to figure it out for a specific tire is to look it up on Tire Rack or similar website.

 

[Norm Peterson]

"Measuring width" wheels are *approximately* a percentage of the tire size section width. It's something like 70% - 75% of the design section width for taller profile tires, say 60, and closer to 85% for profiles below 50. Obviously, there is some rounding off going on, and I don't know offhand if it's to the nearest half inch, or rounded down, or rounded up. I'm going off memory here; there's a spec for this somewhere.

Tire section width varies by 0.2" per half inch variation in wheel width from measuring. This works both ways, with narrower than measuring width wheels pulling the sidewalls in to something slightly less than their section widths on measuring width wheels.

Wheel flanges are typically about half an inch thick to possibly as much as 5/8". Since wheel width is measured inside the flanges, a 10" wheel will measure approximately 11" wide overall. The 10" is the distance between the bead seats, which is what the actual tire dimensions depend on.

For all practical purposes, the width of the tread for any given tire is independent of the width of the wheel it is mounted on. The width of rain grooves is where things start getting complicated - like you can't use the usual assumption of contact patch area equaling tire load divided by inflation pressure any more because the unit load over the contact patch is far from uniform.


Norm

 

[redfirepearlgt]

https://www.discounttiredirect.com/learn/tire-size-calculator


305/30/20 tire
305 is the tread width (305mm/25.4=12.00 tread width)
30 is Aspect ratio for side wall height305*30%/25.4=3.60sidewall)
20 is the only SAE value in the tire size is the rim diameter.

 

[Racer47]

Yes, different brands use different measuring standards.

Thats not true. All DOT approved tires use the same T&RA standards. But the standards have tolerances. The exact same tire could be labeled as a 275 or a 285 if it's actual size was 280.

http://s197forum.com/forum/showpost....7&postcount=25

Plus there are more posts in the rest of the thread if you are interested.

http://s197forum.com/forum/showthrea...67#post2379367

 

[Pentalab]

https://www.discounttiredirect.com/learn/tire-size-calculator


305/30/20 tire
305 is the tread width (305mm/25.4=12.00 tread width)
30 is Aspect ratio for side wall height305*30%/25.4=3.60sidewall)
20 is the only SAE value in the tire size is the rim diameter.

305mm is not the tread width..it's the sidewall to sidewall width..aka section width... when using their.. 'measuring width rim'.

285mm = 11.22" but the actual tread width is only aprx 10".
The other day, I saw 2 x mickey thompson 305 tires, laid on their sides, next to each other. One was a helluva lot wider...and both were on the same width rims. Two different models of a 305mm.. from the same tire maker. The fellow had placed a 4' carpenters level across the wider tire.... and sticking out across to the narrower tire. You could see the huge gap on the narrower tire..to the underside of the level.

Norm, you are correct, my 10" rims measure exactly 11" across, with no rubber mounted.

The +/- .2" of section width... for every 1/2" change in rim width appears to hold true, judging by my experience with the 275 on a 9" rim.. vs the same 275 on a 10" rim. Tread width on the ground stayed the same.

Baffles me why some, say 285's run wide... and some 285's run narrow. One will have wider tread, yet may or may not have the same section width as another brand of 285..which has narrower tread width.

I'm going to try some 305-35-19's next spring, on the rear, on new 19 x 10" rims, but I think I will only see another .5" to .7" wider tread on the ground, vs my 285-40-18's. The 285-40-18's will be shifted to the front.

 

[Racer47]

Baffles me why some, say 285's run wide... and some 285's run narrow. One will have wider tread, yet may or may not have the same section width as another brand of 285..which has narrower tread width.

Tire design is a lot more complicated than you think. There are many factors including the materials, mold shapes, footprint shapes and cost. Give or take a quarter inch, it doesn't matter.

https://www.youtube.com/watch?v=K474RYse9P8

https://www.youtube.com/watch?v=PHdCezJ7Pyw

 

[redfirepearlgt]

305mm is not the tread width..it's the sidewall to sidewall width..aka section width... when using their.. 'measuring width rim'.

285mm = 11.22" but the actual tread width is only aprx 10".
The other day, I saw 2 x mickey thompson 305 tires, laid on their sides, next to each other. One was a helluva lot wider...and both were on the same width rims. Two different models of a 305mm.. from the same tire maker. The fellow had placed a 4' carpenters level across the wider tire.... and sticking out across to the narrower tire. You could see the huge gap on the narrower tire..to the underside of the level.

Norm, you are correct, my 10" rims measure exactly 11" across, with no rubber mounted.

The +/- .2" of section width... for every 1/2" change in rim width appears to hold true, judging by my experience with the 275 on a 9" rim.. vs the same 275 on a 10" rim. Tread width on the ground stayed the same.

Baffles me why some, say 285's run wide... and some 285's run narrow. One will have wider tread, yet may or may not have the same section width as another brand of 285..which has narrower tread width.

I'm going to try some 305-35-19's next spring, on the rear, on new 19 x 10" rims, but I think I will only see another .5" to .7" wider tread on the ground, vs my 285-40-18's. The 285-40-18's will be shifted to the front.

Thanks for that clarification on wall width. You are correct. I miss spoke trying to keep the calculation processes simple.

 

[Pentalab]

I had a slow leak on my pass front oem michelin (tread wear-300) ... on my wife's 2011 Fusion. Bucket loads of tread depth on all 4 tires. Turns out the sidewall was separating from the tread. You could see this ragged indentation line, 360 deg around the circumference. Didn't notice it b4, since it was on all 4 corners. All 4 were starting to separate. Never had that happen..ever. Was right pissed. Replaced all 4 with Nitto Motivos... tread wear - 560. So far so good.