Interesting about the tire width. All are the nominal 140 mm width, but IRC GS-18 lists their rear tire's actual width as 128mm (5.04"), and Dunlop D404 is listed as 5.85". I don't know if they're measuring at the same point. Pirelli lists the maximum width of their 140/90-15 MT66 Route tire as 151mm (5.94"...can that be right? Their 150 is listed as 157 mm). Metzeler does not list a width for their ME 880 Marathon. By the way, Metzeler is a Pirelli brand, bought out in 1986.
A couple of interesting pages on the Metzeler web site about tire pressures and tire upgrades:
http://www.us.metzelermoto.com/web/products/tire_pressures/default.page Quote:Always inflate tires to the correct pressure as indicated in the owner's manual. However Metzeler North America has found the air pressure suggestions listed below will improve mileage and customer satisfaction especially if a emphasis is placed on running the air pressure towards the maximum as stated on the sidewall. Check cold tire pressures frequently. Correct tire pressure is crucial for safe handling. Over inflation may impair ride comfort and reduce the contact patch between the tire and driving surface of the tires. Insufficient air pressure will result in poor handling and cause a tendency for the motorcycle to "wander". In addition, improper and insufficient tire will cause accelerated tire wear, increased fuel consumption, less control and the possibilities for tire failure to due an overload/under inflated operating situation.
Then, they show recommended tire pressures for the ME 880:
ME880 Solo 2 Up Light 2 Up Heavy
Front 38-40 40-42 40-42
Rear 44-46 46-48 48-50
But, the IRC GS-18 has a max pressure of 41 psi.About tire upgrades:
http://www.us.metzelermoto.com/web/products/upgrade_options/default.page Quote:STAGE 1 UPGRADE:
Larger ME880 rear tire on OEM size wheel with no modifications to the motorcycle.
Except they don't recommend an undersized wheel, and both Pirelli's 140 and 150 have a recommended wheel width of 3.0"...our rear wheels are 2.75" wide. Also keep in mind that a bigger diameter tire will give an effective higher gear ratio. The stock GS-18 140/80-15 has an outside diameter of 612mm. The MT66 140/90-15 has an O.D. of 629mm, and the 150/90-15 has an O.D. of 653mm, about 7% bigger for about a 7% reduction in engine rpm and about 7% reduction of power (very rough estimate of the power loss). Top speed may not be helped if our bikes run out of power before they run out of rpms.
By the way, here's what the tire sizes mean for anyone who doesn't know:
140/80-15 M/C 67H
140 = the nominal width of the tire body in millimeters.
80 = the percentage of the tire body height to the width. (70 is a lower tire of the same width, 90 is a taller tire of the same width, etc.)
15 = the wheel diameter in inches.
M/C = Motorcycle tire
67 = Load Index = 677# for these tires
H = speed rating, not over 130 mph for "H"
Also, the last four digits of the DOT code impressed into the sidewall of the tire show the week and year of manufacture.