Hello, can anyone tell me if a 250mm front tire is the same as an 80/90-21 front tire? Thanks in advance.

 

Those are both metric tire sizes. The 250mm (which is only part of the actual size) is almost 10" wide.

 

I'd like to know where you're finding a 250mm front tire listed.
But, you can Try This for future conversion reference.

 

It doesn't have to be a front tire on the front. Suggested, yes...do-able otherwise? Also yes. I have a few bikes with rear tires on the front. My cafe Sportster has front tires on each end.

 

You're correct, you can run a rear tire on the front but it's recommended that it be installed backwards to prevent belt seperation, but his question was;
"can anyone tell me if a 250mm front tire is the same as an 80/90-21 front tire?"

 

In metric tire sizes, the first # is the section width in mm, the second # is the aspect ratio, and the thrid is the rim size.

80/90-21 - 80mm wide, height of tire is 90% of the seciton width, 21 inch rim.
To convert a metric tire size to standard, divide the seciton width in mm by 25.4, then multiply that # by the aspect ratio in % and that will give you your height of the tire from rim to tread, then obviously the rim size. If you want to know the overall diameter of the tire, take the height and multiply it by 2, then add the rim.

Converted to standard -
80/25.4 = 3.15 inches wide
3.15 x 90% = 2.84 inches tall (rim to tread)
21 inch rim
2.84 x2 = 5.68 + 21 = 26.68(this is the overall diameter of your tire)

A 250 mm tire is approximately 9.85 inches wide, or considered a 10 inch tire, the 80/90-21 is 3.15 inches wide.