Anyone know what the diff is betwwen a MH90-21 and a 90/90-21?

 

Look at this link: http://www.dunlopmotorcycle.com/tire...n-tire-series/

The 90/90-21 is 27.1" in diameter by 3.54" wide.
The MH90-21 is 27.28" x 3.25"

 

I think you will find they are nominally the same size, however the 'M' series seem to be Harley specific, with higher load ratings for heavier bikes. When looking at alternative brands always check suitability of load rating.