Looked all over for this but havent found a definitive answer. My 1978 DT250 has a rear tire size of 4.00-18. I've seen some comments that a 4.10-18 is actually a 4" wide tire with a 82% aspect side wall. So it would be a smaller tire than a 4.00-18. Shinko's website does not provide actual dimensions for tires of that size so I'm having trouble proving this. I did order a 4.10-18 from Dennis Kirk because they listed it as a 4.10/100-18. I assumed that meant it was a 100% sidewall height ratio but it appears to be closer to 3.3-3.5" which seems to confirm that the 4.10 = 82% sidewall ratio height. Anyone have a definitive answer on this? Thanks in advance. 

The 4.10 / 100 is an odd way of describing a tyre . My thoughts are that they have combined Imperial and metric measurements .
A 4.10 tyre roughly equates to a 3.50 size . In metric , that would roughly be 100 .
A 4.25 / 85 rear would be closer to a 4.00 , though you'd have to check clearance issues with the swinging arm .

The definitive answer is that, especially with the vintage designations of "4.00-18" there really wasn't any "standard".  Actual measured sizes have always varied from brand to brand and even within brands from tread design to tread design. 

For example, the Pirelli MT43 trials tire in 4.00-18 is HUGE.  Very tall.  Excellent tire.

The tire you got had a designation that was probably "made up" by Dennis Kirk to fit their software or something.  Modern designations that include aspect ratio are always 100% in metric.  And even then there is no definitive exact size you can rely on to say every brand and tread design is going to measure out to the same size.  They just don't.

What's even more maddening is there are actually tires made by the same company in the same tread design and the smaller size numerically is actually LARGER than their next size numerically!   

Check out the master tire thread here if you haven't already:  yamaha-enduros.com/index.php/forums/gene...-tire-thread?start=0

You didn't mention what tread design you are looking for but the closest to stock sizing and looks for the bigger bikes are the Heidenau K67.  They are virtually identical to the original "trials" pattern and size Yamaha used in my experience. Great tires that perform well.  

So they listed it as a 4.10/100-18 but the tire only has 4.10-18 stamped on it. So I'm guessing their tire size input field just needed something so they threw 100 in there. I wanted to try a 4.60-18 as I believe that would be closer to the original diameter but about 1/2" wider but dont want to order the wrong thing again. 
 

The 4.60x18 Shinko 244 will rub slightly on your bike.  I had that size on a 1977 DT400. 

Squirrelly as heck off road in soft dirt/sand too.  Heidenau K67 in 4.00 -18 is more expensive but a much better choice in my opinion.  

Thanks, Mark. Do you agree the 4.10 is a low profile 4.00? Same with 4.60 is lower profile 4.50? I'll check out the K67.

Thank, I missed your first reply here.