[CliffordK (OP)]

I was out shopping today... 
Fortunately I got into a smaller "specialty" store, as I was just totally "blown away" with the answer, and I most certainly would have purchased the wrong item had I gone to a "big box" store.

When does 26x1½ not equal 26x1.5?

[imatfaal]

"speciality store" - as this is a family site perhaps you should keep what you were buying to yourself!!!    []

The only thing I can think of is that you wanted 26 units of say a pint and half; which could be advertised as 26 x 1½ or also as 26 x 1.5.  but when you looked closely the 26  x 1.5 was actually 26 units of a pint and 5 fluid ounces - ie not a decimal place at all.  This could work with pints/fluid ounces, pounds/ounces and in fact most of the imperial measures.  SI measurements should always be ok - unless someone is being very clever and fraudulent

[CliffordK (OP)]

Nope...  Not what I was thinking of...   

Geezer....?

This should be right up your alley.

[imatfaal]

right up gaesar's alley and from a "speciality store" - this gets more sordid as we progress.

[Bored chemist]

My guess is when you are buying cycle inner tubes.I don't know whet the difference is, but that's the only think I ever bought with those sort of dimension.

[Geezer]

right up gaesar's alley and from a "speciality store" - this gets more sordid as we progress.

Now look here Mr Imatewer, I'm getting really tyred of your innuendos.

(After talking with a gentleman assistant, I think BC could be on the right "track".) 

[Geezer]

BTW, when you buy a 2x4 (in the US) what do you really get?

[CliffordK (OP)]

Ok...
Not just the inner-tubes, but the tires themselves.

Apparently bicycle tires are sized by the outer diameter of the tire, rather than the rim diameter like car tires.  So, the larger (wider, and also higher profile) the tire, the smaller the rim diameter.  Of course, it still may vary significantly from the advertized size.

This page has an excellent summary:
http://sheldonbrown.com/tire-sizing.html

Anyway, I seemed to have missed the "issue", as I never had a classic 3-speed with 26" wheels.  Jumping from 24" to 27" (which is a 1-size rim), and then to 700c (I never realized there was also a 700a & 700b).  And, then back to the 26" mountain bikes which all happen to have the same rim size.

Then I was given an older British Made Raleigh for a tune-up...   

Anyway:

26 x 1 ¼ -- 597 mm rim
26 x 1 3/8 --> 590 mm rim
26 x 1 ½ --> 585 mm rim

The modern mountain bikes with 26" tires with decimal widths rim widths from 1.00 to 2.3, and all use a 559 mm rim.  And, they don't even look close.

Actually, I think I may have run into this issue as a kid.  Somehow we had found some aluminum rims for my 24" clincher wheels.  The tires were always exceptionally tight and difficult to get on and off.  Now, looking back at it, my guess is we managed to get 547mm rims, and 540mm tires.

Anyway, so had I purchased a Department Store 26" tire, it would have undoubtedly been a 559 mm tire, and would have been a very tight squeeze on my 590mm Raleigh rim.

[CliffordK (OP)]

BTW, when you buy a 2x4 (in the US) what do you really get?

If you find a "rough cut" board, then you will likely get a 2x4, or something very close.
However, for all other boards, it will be 1½ x 3½.

And, modern mills use a thin-kerf saw so they are unlikely to even cut a true 2x4 before planing, but rather perhaps a 1¾ x 3¾. 

If you happen to purchase an 8' 2x4, it may actually be 92 5/8" long, rather than 96".  When you add a base plate, and 2 top plates, you add about 4 ½", which then gives you 97 1/8".  You then subtract 5/8" for finish flooring and ½" for sheetrock, and you end up with a perfect 96" wall.

[Geezer]

Somewhat related, and sufficiently obscure for me to post, when the UK was converting to decimal currency about forty years ago, some government bright spark put a rather large ad in the papers pointing out that decimalization was not a big deal, and that the model railway fraternity had been using it for many years because;

"They build OO models at a scale of 4mm to the Foot"!

I wish I had kept a copy of it.