Which DOT tire size to get for D/P conversion?

3 replies to this topic
  • dr_penner

Posted April 30, 2002 - 05:55 AM


I am ordering my Baja Designs kit today for my dual-purpose conversion XR650R. My province requires DOT tires and the Pirelli MT 21 that I am interested in do not come in the same size as the stock tires. Which size should I get?

The stock tires are:

Front: 80/110-21 51M
Rear: 110/100-18 64M

The MT-21's come as:

80/90-21 48P
90/90-21 54R

110/80-18 58P
140/80-18 70R
120/90-18 65R
130/90-18 69R

I know what the 21 and 18 stand for. What about the other numbers? I assume one of them is tread width.

Kurt Penner

  • needsprayer

Posted April 30, 2002 - 06:26 AM



I am using Michelin Desert tire up front and Michelin BAJA tire in the rear. The Michelin BAJA tire is a 140, which is equivalent to a Dunlop 120. It's a 6 ply tire that fits fine and provides a large footprint. Not easy to put on yourself though.

These are DOT tires,not cheap and are holding up great. The knobbs are aggressive for dirt riding (90% dirt/10% road). No complaints at all, except they ain't cheap.

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  • billyj7175

Posted April 30, 2002 - 06:35 AM


I could be wrong, but I would stuff as big as a tire as would fit without hitting the muffler on compression. Horsepower does you no good if you can't put it to the ground.

I could be wrong again, but I believe that the first number is the width in mm's and the second is the height in percentage of the width. The third, is obviously the rim diameter. The very last two digits, I have no idea what they mean.


110/100 = 110mm wide/110mm tall (STOCK)

140/80 = 140mm wide/112mm tall

120/90 = 120mm wide/108mm tall (10mm wider, 2mm shorter) This would be close to the stock rear tire


80/110 = 80mm wide/88mm tall (STOCK)

80/90 = 80mm wide/72mm tall

90/90 = 90mm wide/81mm tall

I'm glad you asked this question, as I am getting ready to order some tires myself.... :)

  • dr_penner

Posted April 30, 2002 - 09:17 PM


I think Billy J has got it right. When I spoke with Baja Designs for my order today I asked about this tire sizing stuff and got basically the same answer. The first number is the tread width in millimeters and the second number is the tread depth as a percentage of the first number.

He also pointed out that the Japanese tire makers like Bridgestone (i.e. my stock tires) tend to measure from sidewall to sidewall and therefore endup with a smaller number than do other tire companies who measure across the knobs themselves. So he recommended the 90/90 front and the 140/90 rear.

Whew! Now that that's over, does anyone know whether the D.O.T. stamp of approval is written anywhere on the MT21 tire carcass? That would go a long way to pleasing the officials when the time comes. My province's regs stipulate that nowhere on the tire can it say "for off-highway/competition use only". What does the MT21 say?

Kurt Penner

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