170/80/15 versus 180/70/15. The first number is the section width in millimeters, meaning how wide the tire is across the tread. The second number is the aspect ratio, meaning how tall is the sidewall from rim to tread as a percentage of the section width. The last number is the rim diameter in inches.
On our bikes, the stock rear is 180/70/15, meaning the section width (tread width) is 7.09 inches or 180 millimeters. The aspect ratio is 70% of 180, meaning 126 millimeters or 4.96 inches. So the total tire height from tread on the ground to the tread on the top is (diameter, including rim) is 24.92 inches or 633 millimeters.
By changing to the tire I use, I am decreasing the section width (180 millimeter to 170 millimeter) and increasing the aspect ratio (from 70% to 80%), thus making the tire taller in overall diameter. The following chart is from Senior Member Whiting57:
Revs per mile_____784.49______809.27______774.99
Now, how does this translate into rpm at speed. Take the revs per mile listed (it is actually the engine revolutions at the crank per every 10 mph) and multiply it by the speed you are looking for.
On the stock rear tire, at 70 mph, is 7 X 809.27 or 5,664.89 rpm; the engine can handle this sustained rpm without problem (as an aside, my son's 82 KZ550 LTD is turning over 6,000 at 70 and I have held 90+ mph or 7000+ rpm for miles on end trying to keep up with big bore bikes on my V9D with zero problem, but it took me a lot of getting used to). With the 170/80 tire, it is 5491.43 rpm at 70 mpg. 170 rpm doesn't sound like a lot, but at speed, to me, it makes a difference. The 200/70 is 5424.93 rpm at 70 mph. There are those who put a 230/70 and 240/70 tires on the rear.
There are the ScootWorks front and rear pulleys. The premise here is like on a multispeed bicycle; the larger the front sprocket (more teeth), the fewer cranks of the pedals required to maintain a given speed, and on the rear, the smaller the sprocket (fewer teeth) for fewer cranks to maintain a given speed. According to the Scootworks site (http://www.scootworks.com/shop.cgi/p...223118943.2313
), the front pulley is up two teeth over the stock front pulley and the rear is down four teeth over the stock rear pulley for a 5.9% or 6.1% reduction in rpm at speed. You can combine pulleys and tire sizes to suit your needs and budget. Thus, with the stock tire size, at 70 mph, would be 809.27 X .941 (1 - .059) X 7 or 5330.66 rpm for a front pulley and 5319.33 rpm for a rear pulley. Front pulley and 170/80 tire would be 5167.43 rpm at 70 mph, rear would be 5156.45 rpm at 70 mph. Some do one or the other pulley mod, some do both.
As for modification, none was needed for the 170/80, but from what I have read, for the 200 and up, modifications are needed (inner fender, pulley change, new belt).
As for drive-ability, I've noticed no negative. Take off is just as good and I can run higher speed in each gear before I feel she needs to shift. Works much better in the twisty two-laners; I can run 40 to 45 mph in second and not feel I am hurting the engine yet still have plenty of pull to not be a snail to those behind, even two-up.
Price wise, the pulleys are kind of expensive (I would suggest only purchasing new pulleys; from what I have read, the older ones had an issue with a belt squeal) and the 200+ tires are spendy. The 170/80 are cheaper than the 180/70, handle just as well if not better, and were the cheapest way I could find to drop rpm for now.
Those with experience with different combos of pulleys and tire size, chime in with your opinions as to how these mods helped make the best midsize cruiser on the market even better.
Hope this helps.