Are 26 x 1 3/8 and 26 x 1.4 tyres interchangeable?

[Chris S]

26 x 1 3/8 tyres are hard to source while 26 x 1.4 tyres are fairly common.

There is only 25/1000 of an inch difference between them (1.4 - 1.375)

I doubt if it's possible to manufacture flexible rubber tyres to this tolerance so are they effectively the same size and interchangeable?

 

[Dan B]

26 x 1 3/8 tyres are hard to source while 26 x 1.4 tyres are fairly common.

There is only 25/1000 of an inch difference between them (1.4 - 1.375)

I doubt if it's possible to manufacture flexible rubber tyres to this tolerance so are they effectively the same size and interchangeable?

26 x 1 3/8 is a "590" tyre in the ETRTO naming scheme. 26 x 1.4 is more likely a 559. That's a 31mm difference in rim size, so I would guess the answer is "no"

 

[tyred]

Definitely not interchangeable but you should have no real difficulty in finding 26 x 1 3/8. Any number of Ebay sellers list them, SJS have a reasonable selection, even Halfords stock them.

 

[andrew_s]

Fractional inch sizes are based on the external diameter of the tyre, not the size of the rim.
To get the rim diameter you subtract twice the tyre height from the tyre diameter.

26 x 1 3/8" = 25.4 x (26 - 2x1.375) = 590mm
26 x 1 1/4" = 25.4 x (26 - 2x1.25) = 597mm

Decimal inch sizes like 1.4 are all for 559mm MTB rims. This is the size for a 26 x 2" tyre.
26 x 1.4 is a 1.4" wide tyre that fits on a rim for a 26 x 2" tyre
A 26 x 2.0" size is the only MTB tyre size that actually measures 26" in diameter.