Diameter difference:110-90 and 120-90?

My stock rear was 4.00-18, so now I run the metric equivalent which is 110-90-18. I tried to measure the wheel diameter and it appears to be 26" as close as I can measure.

I was wondering how much it would increase going to 120-90-18?

If I understand the way they spec them, the diameter would increase about 0.4" more on each side or a total of maybe 0.8" diameter across the wheel.... which is about 3%.

Is there a table that shows the diameters on various tire sizes?

120 is the width..it just means a wider tire not taller ...am i wrong???

120mm its a metric thing.
so 5 more mm Tyre each side.

Yes a 120/90 is a taller tire than a 110/90 as well as a wider tire. 90 is the aspect ratio meaning 90% as high as wide so going by the math if I'm thinking right the 120 would be about 9mm higher tire so 18mm would be the combined difference or about 0.7" difference between the two tires. A stock 4.00-18 is close to a 120/90 in diameter, but narrower. A 110/90 is close to the right width, but too low. So the conclusion I've made after trying all these sizes is either use a 120/90 if you like the wider tire, or a 4.00-18 stock size. A 110/90 is just not right at all.

You can not go by the tire size, there is no consistency, every manufacturer is different.

A 120/90/18 of one brand will not have the same dimensions as the same size tire of a different brand. Some of the tire manufacturers websites will give you the dimensions of the various size tires in different models (a 120/90/18 tire is not even consistent in size across the same manufactures model line).


It wouldn't surprise me if you found a 110/90/18 tire from one manufacturer that is larger than another manufacturer's 120/90/18.


When I was playing around racing sports cars, I went to the dealer and measured each tire until I found four that matched. And this was one model tire from a specific manufacturer.

Crazy, isn't it?

RonKZ650 wrote:

A stock 4.00-18 is close to a 120/90 in diameter, but narrower.

I found that out the hard way. My bike came stock with 4.00-18 tire and 16/38 gears which is WAY too low. I bought a new chain and gears of 17/38, but the RPM are still same as original setup because the 110-90 tire I have to use is smaller diameter. I ran a 120 tire once but the center strip burned out faster because the rim is too narrow and that pinches the sidewalls in and pooches out the center of the tread so it isn't the correct profile. The 100 tire got considerably better tread life.

These metric tires really are not the right fit.

steell wrote:

It wouldn't surprise me if you found a 110/90/18 tire from one manufacturer that is larger than another manufacturer's 120/90/18.

I don't understand how that's possible because the reference at the top of this forum section says that the "110"mm is the tire width and the 90 is the percent aspect ratio (the percent of the width that the height of the tire is). If you specify the width and the aspect ratio, that should define the height of the tire. Add two heights to the rim diameter and you should get the wheel diameter.

Based on my tire, it works: the rim diameter is 18", the tire is 110/90 which means the width is 110mm = 4.3". Multiply times 90% and you get a height of 3.9". Twice that is 7.8" and added to the 18" rim diameter you get 25.8" which is the diameter I measured on my tire.

Take a look at Dunlop's website for an example, they list the actual width and height of the specific tires, and you'll see. It's been mentioned here on KZR a bunch of times by various people, usually when talking about how big a rear tire you can run.

Think of the tire numbers as a "class" and the different diameters/etc as variations within that "class".

Dunlop D404:
100/90-19 - 2.15 - 25.82 - 3.90

Dunlop GT501:
100/90-19 - 2.15 - 26.12 - 4.05

Tire size- rim width- diameter- section width


And that's two tires of the same size from the same company.


www.dunlopmotorcycle.com/tirecatalog_category.asp?id=3



So lets take a look at the rear.

Dunlop 404
120/90-18 - 2.75 - 26.44 - 4.76

Dunlop GT501
120/90-18 - 2.75 - 26.50 - 4.75

Closer on the rear.


GT501 is not available in 110/90/18, but the 404 is.

110/90-18 - 2.50 - 25.51 - 4.41

Compare that to your measurements.

Differences can be even larger when comparing two different manufacturers.

I had nothing to do with it being this way, it's just something I ran across years ago

Steell is correct, you can't depend on the numbers written on the sidewall, you have to measure the actual tyre. I run "180" width tyres, some are 180mm, some are 190mm, some are 200mm!

And yes, Steell, sometimes I pump them up...:laugh: :blush:

I have 2 new Michelin 100/90 19 front tires in my rack. One, the old model A48, is MUCH wider and smaller diameter than the other - a new Macadam 50E. Frankly, a 110 sounds very narrow to me as a rear tire on these bikes. I used the Michelin 130/80 18 M48E tires on the rear of my KZ650 with great success for years (until they stopped making them). I now have a 120/90 18 Michelin Macadam on the rear but I may go back to a 130 next change. The center of these tires didn't show unusual wear, but I don't do burnouts. However, I do push it a bit in the turns and the extra width seems to help. Ed

Mines a Cheng Shin HI-MAX 100-90-18 and it definitely does not look as tall as the stock tires did which were 4.00-18.