tatfiend 

I have an older pair of Vittoria Tecno Twin Tread kevlar bead tires still in the boxes that I bought some years ago. The boxes and tires are marked 700 x 25C.

I finally installed one on a rim today. After pumping it up I noticed that it appeared to be much narrower than I expected. I measured the total width, sidewall to sidewall, with a caliper. It measured 20.5mm, almost 20% narrower than claimed. Too narrow a tire for me to want to use at my current weight taking into consideration the condition of many local roads.

Why cannot bicycle tire manufacturers give accurate measurements for tires? The amount of discrepancy I found seems ridiculous. These are appreciably narrower than my old Conti Grand Prix 700 x 23C tires which themselves are narrower than claimed.

Any tire make recommendations from others for tires that actually match the listed widths when installed?

StephenH

This may be part of the answer. From:
https://www.sheldonbrown.com/tire-sizing.html

"Competitive pressures have often led to inaccuracy in width measurement. Here's how it works: Suppose you are in the market for a high performance 700 x 25 tire; you might reasonably investigate catalogues and advertisements to try to find the lightest 700-25 available. If the Pepsi Tire Company and the Coke Tire Company had tires of equal quality and technology, but the Pepsi 700-25 was actually a 700-24 marked as a 25, the Pepsi tire would be lighter than the accurately-marked Coke 700-25. This would put them at a competitive advantage. In self defense, Coke would retaliate by marketing an even lighter 700-23 labeled as a 700-25.

"This scenario prevailed throughout the '70's and '80's. The situation got so out-of-hand that cooler heads have prevailed, and there is a strong (but not universal) trend toward accurate width measurements."

tatfiend 

I have read that before as well as Frank Berto's similar comments in his book Upgrading Your Bike. I just never realized how extreme it could get. A claimed 25C that is actually about a 20C to 21C seems totally ridiculous IMO.

Bill Kapaun

How much different is the inflated tire diameter from 672MM?
622+25+25
Sidewall to sidewall isn't the measurement! It's bead seat to outside center of the tread.

StephenH

As best I can tell from Wikipedia and Sheldon's site, it is the actual width and not the depth that is measured on bicycle tires.

When I worked with truck tires in years past, I know it was the depth there- a 1020 tire was 10" depth on a 20" rim.

Bill Kapaun

So all bicycle computer calibration sheets would have to assume all tires had identical profiles?

tatfiend 

The numbers with the computers are an average for true tire width as I recall. I have always read that for greatest accuracy you want to do a rolling test to get the most accurate number for the computer.

Both Frank Berto's writing and Sheldon Brown's regarding bicycle tire sizing seem to clearly refer to width. Road bicycle tires are not made in low profile versions that I have ever heard of. All that I have looked at seemed to have virtually identical profiles.

For car and motorcycle tires, at least metric ISO listings. ie a 185/70 x 14 is a 185mm wide, 70% aspect ratio tire for a 14" rim. The first number is width.

Doohickie

As long as the tread is the correct distance from the rim, the computers are good. But also remember that bike computers are basically just toys.... "not legal for commercial transactions". They are approximate.

It's kinda like dimensional lumber..... ever measure the cross section of a 2x4?

mawtangent

In my experience some claimed tire-widths have been spot-on (like my Michelin Dynamic, Maxxis Xephyr and Marathon Plus tires, all 700 X 25). I bought an '86 Schwinn Traveler with old Specialized touring tires that read 27 x 1 1/8, they measured 1 inch wide on the bike. I liked them being 1 inch wide, so when I ordered a new set of 1 1/8 inch Panaracer Pasela Tourguards I was hoping to get 1 inch wide tires. The Panaracers measured true 1 1/8. So that's what I get for trying to second-guess the claimed measurements (note: the Panaracers worked out okay and no flats so far).

I've read before that rim width (think unusually wide or narrow rims) can lead to discrepancys with claimed tire widths, which makes sense.

rogerstg

That's why you are supposed to actually measure them if you are interested in accurate distance and speed. Most computers allow you to enter the actual circumference.

CCrew

Naa, they're screwed up there too. I have 36x13.50-15's on my Jeep, and they're 36.9 tall. 37 x 13.50's are 36.7 tall. But the 37's are $50 each more :-)

I ran into the same thing with a set of 28mm's from Performance we just bought. They won't even fit between the calipers, and we took 28's off


-R

Retro Grouch 

Can't be done because rim width affects installed tire width and 700c rims aren't all the same.

I run 700 x 28c Gatorskins on two different bikes. They actually measure 27.2mm with Velocity Dyad rims and 26mm with Velocity AeroHead rims.

Pat

Sheldon Brown's explanation sounds pretty plausible doesn't it? Well, Sheldon usually made sense.

I have noticed what that frequently 2 tires of the "same size" did not appear that way. I have not gotten out calipers and measured them inflated.

I have always figured that for some odd reason each and every tire manufacturer had their own idea of what a "mm" was.

NormanF

ETRTO and ISO brought about uniform tire sizing standards so the situation for cyclists is a lot easier than it once was. All you need to do is determine the dimension of a tire to a given rim width. If they don't match, its not compatible.

tatfiend 

I believe that should be bead seat diameter rather than width in your post. Some people run wide tires on relatively narrow rims and vice versa such as mountain bikers.

On the same relatively narrow Trek Matrix rim, intended for a drop bar road bike, a Conti 23C measured about 22mm wide and the Vittoria 25C measured 20.5mm. The Conti at least was in the ballpark as regards listed width versus measured but the discrepancy with the Vittoria was ridiculous IMO. Normally a buyer of a 25C tire is not going to be a weight weenie looking for ultimate performance. They buy a slightly wider tire because they want a wider tire.