Tyre sizes
The markings on tyres cause untold
confusion. Did you know that some tyres marked 28" are actually much less
than 27 inches diameter, or that 26×1.5 and 26×1½ are two completely different
sizes!
Most tyres are
now marked according to the International Standards Organisation (ISO) in
addition to more familiar markings. The ISO size comprises two numbers
separated by a dash. The three-digit number after the dash is the most
important. This is the bead diameter at which the tyre fits onto the rim. Check
the bead diameter of the tyres on your bike. If you’re ever in desperate need
of a tyre and can find one with the same bead diameter, it will probably fit
even if every other thing about it is different.
That initial
two-digit number is the tyre section and roughly indicates the tyre width. Its
actual width is normally less, but varies according to how the tyre is designed
and the width of rim to which it is fitted. But the height of the tyre usually
corresponds quite closely to tyre section, so overall diameter approximately
equals the bead diameter plus twice the section. That’s how the diameter and
circumference (Circ. – equals distance rolled in one turn of the wheel) are
calculated in the Tyre Size Table. These figures are useful for setting your
cycle-computer and precisely calculating gears.
Half of the bead
diameter (i.e. the bead radius) corresponds with the distance from the centre
of the wheel to the centre of a brake block. By comparing this dimension you
can see if the brakes will reach when a different size of rim is fitted to your
bike. Switching from 630 to 622 for example, needs another 4mm of brake reach.
Imperial, Metric & American
Sometimes you
can’t find any ISO size on a tyre, only the old familiar markings. Although
they’re familiar they can be very confusing. Tyres of same size come with lots
of different markings: e.g. 28×1¼×15/8 or 28×1¾×1¼ or 700×32C are
all 32–622. Even worse, the same or very similar markings can sometimes be
found on completely different sizes of tyre: e.g. 26×1½ is a whole inch bigger
than 26×1.5. So use these columns to convert yourself to ISO and then have
nothing more to do with those old markings!
Refer to the
column headed Imperial, Metric or American in order to work out what actual
(ISO) size you have. The first two headings need no explanation. The American
column is for tyre sizes originating in the USA and where US markings may
confuse. Note that American and Japanese branded road tyres tend to be smaller
than described and often lack any ISO markings.
Tyre–rim compatibility
Two tyres with
the same ISO markings are completely interchangeable, even if one seems to be
metric and the other is also marked in inches: e.g. 650B and 26×1½ are both
38-584 and should be marked accordingly. Look for the ISO numbers and you
always know exactly where you are. Some rims are also now marked according to
ISO with their bead diameter and internal section (width where the tyre fits):
e.g. 622–17 for the size usually fitted to touring and hybrid bikes. But some
manufacturers give the overall width instead. Subtract 6mm from overall width
to estimate the rim section, or simply measure the distance between the
flanges. If the rim is not marked with its bead diameter you can read this from
the tyre – it’s the one dimension they have in common.
The ideal
relationship between tyre and rim section is about 1.8 to 1, but any tyre from
1.4 to 2.2 times should fit – always provided that the bead diameters
correspond.
Nowadays nearly
all rims have hooked flanges (required to retain folding tyres) that will hang
onto tyres up to three times their width. This increases the cushioning effect:
you get more comfort out of the same tyre at the same pressure when you fit it
to a narrow rim – yet it still rolls just as easily. On the other hand it
increases the stress on the sidewall of the tyre. Mountain-bike tyres are
designed to take this extra stress but road tyres are not. Another reason not
to exceed the 2.2 limit on road is that a more bulbous tyre can deflect
sideways with a noticeable effect on control and roadholding. Conversely,
fitting a tyre of less than 1.4 times the rim section will result in a harsh
ride and more rolling drag than a larger tyre at the same pressure.
Tyre Size Table
The table below
lists most of the sizes of tyre that are likely to be found in the UK, but some
common tyre sections are omitted to save space. Where you see a block of two or
more compatible sizes, having the same bead diameter, you can assume that a
variety of intermediate sizes exist that will also fit the same rim. For
example: between 18–622 and 25–622 there are tyres of 19, 20, 22 and 23mm
section. Sizes highlighted in white seem most likely to be available in the
long term. Anyone specifying a new cycle should try to avoid other ISO
diameters.
|
ISO
|
Imperial
|
Metric
|
American
|
Tyre Diameter
|
Circumference
|
|
sect–dia
|
|
|
|
(mm)
|
(in)
|
(m)
|
|
40–635
|
28×1½
|
700B
|
|
715
|
28.1
|
2.25
|
|
25–630
|
27×1
|
|
27×11/8
|
680
|
26.8
|
2.14
|
|
32–630
|
27×1¼
|
|
27×13/8
|
694
|
27.3
|
2.18
|
|
18–622
|
|
700×18C
|
700×20C
|
658
|
25.9
|
2.07
|
|
23–622
|
|
700×23C
|
700×25C
|
668
|
26.3
|
2.10
|
|
25–622
|
|
700×25C
|
700×28C
|
672
|
26.5
|
2.11
|
|
28–622
|
28×15/8×11/8
|
700×28C
|
700×32C
|
678
|
26.7
|
2.13
|
|
32–622
|
28×15/8×1¼
|
700×32C
|
700×35C
|
686
|
27.0
|
2.16
|
|
37–622
|
28×15/8×13/8
|
700×35C
|
|
696
|
27.4
|
2.19
|
|
47–622
|
|
700×47C
|
|
716
|
28.2
|
2.25
|
|
32–597
|
26×1¼
|
|
|
661
|
26.0
|
2.08
|
|
28–590
|
|
650×28A
|
|
646
|
25.4
|
2.03
|
|
37–590
|
26×13/8
|
650A
|
|
664
|
26.1
|
2.09
|
|
28–584
|
|
650×28B
|
|
640
|
25.2
|
2.01
|
|
40–584
|
26×1½
|
650B
|
|
664
|
26.1
|
2.09
|
|
20–571
|
|
|
26×¾
|
611
|
24.0
|
1.92
|
|
23–571
|
|
|
26×1
|
617
|
24.3
|
1.94
|
|
40–571
|
|
650C
|
|
651
|
25.6
|
2.05
|
|
25–559
|
|
|
26×1.0
|
609
|
24.0
|
1.91
|
|
32–559
|
|
|
26×1.25
|
623
|
24.5
|
1.96
|
|
40–559
|
|
|
26×1.5
|
639
|
25.2
|
2.01
|
|
47–559
|
|
|
26×1.75
|
653
|
25.7
|
2.05
|
|
50–559
|
|
|
26×1.9
|
659
|
25.9
|
2.07
|
|
57–559
|
|
|
26×2.125
|
673
|
26.5
|
2.11
|
|
28–541
|
|
600×28A
|
|
597
|
23.5
|
1.88
|
|
37–541
|
|
600×35A
|
|
615
|
24.2
|
1.93
|
|
32–540
|
24×13/8
|
|
|
604
|
23.8
|
1.90
|
|
28–520
|
|
|
24×11/8
|
576
|
22.6
|
1.81
|
|
37–507
|
|
|
24×1.5
|
581
|
22.9
|
1.83
|
|
47–507
|
|
|
24×1.75
|
601
|
23.7
|
1.89
|
|
37–501
|
22×13/8
|
|
|
575
|
22.6
|
1.81
|
|
37–489
|
|
22×13/8
|
|
563
|
22.2
|
1.77
|
|
28–451
|
20×11/8
|
|
|
507
|
20.0
|
1.59
|
|
35–451
|
20×13/8
|
|
|
521
|
20.5
|
1.64
|
|
28–440
|
|
500×28A
|
|
496
|
19.5
|
1.56
|
|
32–440
|
|
500A
|
|
504
|
19.8
|
1.58
|
|
28–406
|
|
|
20×11/8
|
462
|
18.2
|
1.45
|
|
37–406
|
|
|
20×1.5
|
480
|
18.9
|
1.51
|
|
47–406
|
|
|
20×1.75
|
500
|
19.7
|
1.57
|
|
54–406
|
|
|
20×2.0
|
514
|
20.2
|
1.61
|
|
32–369
|
17×1¼
|
|
|
433
|
17.0
|
1.36
|
|
28–355
|
|
|
18×11/8
|
411
|
16.2
|
1.29
|
|
40–355
|
|
|
18×1.5
|
435
|
17.1
|
1.37
|
|
47–355
|
|
|
18×1.75
|
449
|
17.7
|
1.41
|
|
37–349
|
16×13/8
|
|
|
423
|
16.7
|
1.33
|
|
32–340
|
|
400A
|
|
404
|
15.9
|
1.27
|
|
37–340
|
|
400×35A
|
|
414
|
16.3
|
1.30
|
|
47–305
|
|
|
16×1.75×2
|
399
|
15.7
|
1.25
|
|
47–203
|
|
|
12×1.75
|
297
|
11.7
|
0.93
|
Recommended pressures
All tyres are
marked with a pressure. Sometimes this is an absolute maximum that nobody
should need to exceed, or sometimes only a recommended maximum that may be
exceeded by heavier riders (e.g. tandemists).
Off-road tyres
are usually marked with a pressure range, from the minimum that will support a
rider of average weight to the absolute maximum. Do not exceed the maximum when
tyres are marked with a range