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A lot of us have done it. Between the tire and the aftermarket front pulley, my speedometer is spot on, well, one mph over. I'm trying to remember, I believe the tire was a little taller? Maybe it was a little fatter. We had this discussion a lot back around 2010, but I've slept since then. You should be able to find some old threads on it. BTW, mine is a classic, but the rear tire is the same.
 

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The tire is skinnier and taller than stock. I ran it on my 900 Classic and it helped a tiny bit with RPM at speed.

It will not hurt anything, as it's within an 'acceptable' difference.

FYI tire size works like this:

width in mm/% of width for height R(stand for radial) wheel size

A 180/70r15 is 180mm wide, 70% of 180 mm tall, on a 15" wheel.

Therefore 170 mm wide / 80% of that tall on a 15" wheel.

The sidewall height is on each side of the wheel too.
 

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Discussion Starter #5
Thank you. I understand what tire sizes mea. I was just curious is all why go skinnier and taller. I like the fatter tires myself. But was curious if it was for a specific reason. Like on my gsxr 1000 I went with a 180/50-18 which is generally a 600 sport bike tire for better tighter cornering. But a cruiser is different.
 

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Why would someone put a 170/80-15 on an 08 900 custom?? When the factory calls for a 180/70-15!!
I just put new Michelins on my 9. Went with the stock 180-70-15. The guy at the shop put the two together(170-80-15 off the rim) there wasn't much of a difference on height.
 

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It doesn't look like much difference on most tires but you can feel the difference. It works out to 2.2 mph difference for the 170/80/15 over the 180/70/15. Add the Barons front pulley for an additional 4.6 mph. Total for both is an additional 6.8 mph at the same RPM in 5th gear. See this link to calculate any tire and pulley size http://www.gearingcommander.com/
 

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Discussion Starter #10
Well I got out on the hwy today and was running about 80 jumped up to 90 once. My boss was in his truck and said we were doing about 70 and got up to 80 once. So I know it's off at those speeds. Thing is the % the speedometer is off in any motorcycle is exponential. Its never cut n dry. If you are off 1/2 a mile at 5 mph you are not necessarily off 1 mile at 10. It changes as the tire heats and cools, expands and contracts.
 

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Sounds like a lot of variables going on there with two vehicles and two sets of worn tires and two speedo's that may be off and not being able to tell exactly your vehicles speed. I think the gearing commander web site is probably fairly accurate at a specific rpm in a specific gear for a specific tire ....mine is exact to radar speed signs.
 

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Discussion Starter #12
Sounds like a lot of variables going on there with two vehicles and two sets of worn tires and two speedo's that may be off and not being able to tell exactly your vehicles speed. I think the gearing commander web site is probably fairly accurate at a specific rpm in a specific gear for a specific tire ....mine is exact to radar speed signs.
He has new tires. Speedo maybe. My bike definitely off. I hope to be radar accurate one day. Lol.
 

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If you like the fat look, 200/70 - 15's fit ok on the Custom. Obviously, 70% of 200 makes them a little taller than stock as well, so's you can get a little closer to true speed with them. But then there is the money issue ...
 

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To the OP
The stock size as far as I could find was through the dealer only, and at $200 ++
Just not worth it.
 
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