26x1.25 tire on a 26x1.38 rim
 

Will a 26x1.25 tire will fit on 26x1.38 rims?
also what's the difference between the two tires?

Short answer: 26x1.25 is a different, incompatible size from 26x1 3/8. Actually, 26x1 3/8 is different from 26x1.38.

Long answer:

https://www.sheldonbrown.com/tire-sizing.html

If it turns out you need 26x1 3/8 tires, I can point you in the right direction.

my rim is 1.38 and want to install 1.25

What tics is saying is that before you can address the width, you need to figure which 26” rim you have. 26” are not all the same diameter. You need the etrto of the existing tire first.

You’ll probably still have an issue with a narrower width tire than the rim.

John

Where are you coming up with the 26x1.38 for the rims? Is it actually written 26x1.375 and you are rounding up or is it 26x1-3/8?
26x1.375 is a 599 mm BSD
26x1-3/8 is a 590 mm BSD or possibly a 597 mm BSD

If you have the old tire that went on that rim, then look for the ISO or ETRTO number of the tire size. Then you'll know what the rim BSD is if that tire fit the rim correctly. The ISO number is many times in the very small print that is embossed or stamped on the tire. Even smaller than the inflation pressure recommendations. You may even find an ISO size printed on the rim.

You can change the width of the tire for that rim quite a bit within certain limits. But the BSD of the tire and rim must match.

I'm assuming you went, read and understood the information I posted earlier, in which case, no, your rim is too wide for that tire.

This^^^^ Use 26x1.50 or get a rim that fits 26x1.25.

Stumbled on this other article that Sheldon had written specifically on 26" tires. I'd seen it before but had forgotten it. Turns out there are at least five different BSD's which probably makes them the most confusing size to have to buy for if you don't know the BSD and are only going by the bogus tire size.

https://www.sheldonbrown.com/26.html

While Iride01 's has offered the most important information that IF THE International Standard BEAD SEAT DIAMETER Number marking IS NOT EXACTLY THE SAME, THAN THE TIRE WILL NOT FIT !!!!!

Iride01 did leave out the all important information for folks who may find themselves with old bikes WITH OLD TIRES THAT PRE-DATE THIS International Standard BEAD SEAT DIAMETER Number marking!!! Chances are Great that your tires likely won't have this IF You Have A BICYCLE from 1968 that has the original tires or the replacement tires were installed Fifty Years AGO , BEFORE THIS International Standard BSD number marking in millimeters appeared on Bicycle Tires.
***** THIS CAUSES A GREAT DEAL OF CONFUSION WITH Bicycle Newbees THAT FIND SOMETHING ANCIENT IN GRANDMA's BARN.***********
It is really very simple as IRIDE01 pointed out.

Basically, the super-ancient cruiser bicycles (just called "ordinary" bicycles way back then) are the 26 WHEEL (559mm bead seat diameter) THIS WAS WHAT ADOPTED AS THE STANDARD FOR Late Seventies & Eighties Era MOUNTAIN BIKES.


What was known as Your "LIGHTWEIGHT" (26) bikes in the super ancient past ( TYPICALLY 3 speeds, and single speeds from the 1940's thru the 1990's, and from the 1960's-1970's era you have 5 SPEEDS, and some 10 SPEEDS from the sixties & dept store 10 speeds of the 70's to maybe 1983 or 1984)
----------------THESE "Lightweight" (26) bikes ARE WHAT will HAVE the 26 x 1 3/8 TIRES. ***notice that I said 26 x 1 3/8 TIRES, as in plural, BECAUSE THEY ARE TWO DISTINCTIVE AND COMPLETELY DIFFERENT ( you cannot interchange them!!) -------- the 597mm chicago SCHWINN ONLY 26 x 1 3/8 (597mm) ----AND--- the 590mm common EVERYBODY ELSE 26 x 1 3/8 (590mm) tire.
Now you're probably thinking, well how the hell did folks know the difference between these Tires BEFORE the INTERNATIONAL BSD millimeter marking that became standard in the Late Seventies. WELL ON THE OLD Schwinn tires, they would usually say *** To Fit On English EA-1 rim & Schwinn S5 / S6 Rims ONLY*** Now that English application (597mm) is so obscure that it was rare and abandoned in the 1950's, so for all practical purposes it (597mm) is the chicago SCHWINN ONLY SIZE!! The KENDA k23 26 x 1 3/8 (597mm) TIRE IS THE ONLY CHOICE TODAY FOR THE chicagoSchwinn TIRE!!
-----THERE ARE AT LEAST Ten MANUFACTURERS WHO CURRENTLY PRODUCE THE "Common EVERYBODY ELSE" (590mm) 26 x 1 3/8 590mm TIRES!!---


So how the heck can you typically tell a CRUISER/Mountain Bike 26 INCH (559mm) Tire from those 26 INCH (590 mm) and (597mm) tires on ancient"lightweight" 3 speeds/single speeds/and some 10 speeds/5 speeds ???? How can you tell easily from a distance??
ANSWER: well first off, the CRUISER/MOUNTAIN BIKE tire typically has a MUCH WIDER TREAD FOOTPRINT, as well as A TALLER SIDEWALL PROFILE FROM THE PERIMETER OF THE RIM............The Cruiser/Mountain Bike wheel (559mm) IS ALMOST ALWAYS CONSIDERABLY WIDER too and thus the tire is not as Narrow as what you have on the COMMON-everybody else(590mm 26 x 1 3/8) or the chicagoSCHWINN ONLY (597mm 26 x 1 3/8)......

Now as RIDE01 and others already mentioned, the Sheldon Brown site link will have about everything you might need to know about all the other possibilities.
Yeah, there are others. Schwinn had a proprietary size on their middleweight cruisers from the 1950's, 1960's and maybe into the seventies (571 , I think it was but you won't find this tire in any retail store like Wallyworld or TarJay.......there are a couple of large beach cruiser-bike parts/bike shops outta So Cal that have huge web stores that have had leading Korean/Taiwan tire manufacturers reproduce those, and if you have a such an old Schwinn cruiser that needs those, the availability is really good, as those remain very popular bikes among the cruising - caber crowd.

**** THOUGH THE common-Everybody Else (590mm) 26 x 1 3/8 TIRES and the chicago Schwinn ONLY (597mm) 26 x 1 3/8 TIRES will not interchange, THE 26 x 1 3/8 INNER TUBE FUNCTIONS PERFECTLY FOR BOTH OF THOSE.


The reason that my discussion was centered specifically on the TWENTY SIX sizes: 559mm (cruiser & old mountain bikes) --and the ancient lightweight 3 SPEEDS 5speed/some 10speed from EVERYBODY ELSE-common (590mm) and ancient lightweight 3 speed/single speed/5 speeds/a very few 1960's 10 speeds from chicagoSCHWINN (597mm) IS BECAUSE THOSE ARE LIKELY WHAT YOU WILL ENCOUNTER OUT IN THE WILD IF YOU'RE RIDING AN OLD BICYCLE WITH HANDBRAKES & GEARS and 26 INCH WHEELS!!!! That is probably all you might need to know.

Essentially in situations like this one must exactly match the tire. If it hasn't been changed from the original tires than Vintage Schwinn gave a really excellent write up on the subject.

The 26" size is kind of a nightmare but that above write up and what the late great Sheldon Brown has written is excellent.

If you haven't figured it out yet, a 26 x X.XX (decimal) tire and rim is generally not the same as a 26 x 1 X/X (fractional) tire and rim, even if the decimal and fractional values seem to equal the same - 26x1.5 is NOT the same as 26X1 1/2, 26 x 1.375 is not the same as 26 x 1 3/8.

If your rim says 26x1-3/8 then you no tire with a decimal (26 x X.XX) will fit. If your rim actually has 26 x 1.38 stamped then it is likely it will work with standard 'mountain bike' or 'cruiser' 26" tires with a 559 mm bead seat diameter. However, I have never seen 26 x 1.38 as a size printed on a tire or rim, but there is an obsolete size that may be compatible with also obsolete 26 x 1.25 from the same era IF that's what you have. Any modern 26 x 1.25 tire will not fit.