So, I went out and measured the rear rim width of my 77 CB750F Since the tire is still mounted, I could only get the outside dimension of 2.7 inches. Subtract about 1/2 inch for both sides of the rim and you get a 2.2 inch bead width for the tire seating carcass. I think the standard size is actually 2.15 inches.
Whatever tire you get must be engineered to perform with this size rim. (Unless, of course, you change the rim width size.)
According to Dunlop, the rim width for a GT501 in a 130 size should be 3.00 inches. The 120 should have a 2.50 inch rim width. So, these tires are not a proper fit on this rim. I couldn't find any 501 tires that would fit properly on the 750 F stock rim. However, the K70 front 3.25/19 and the K81 TT100 rear do specify fitment onto 2.15 rims and should work well on the 750F if you want the Dunlop brand.
Then I looked at the Bridgstone site for the BT45 18 inch tires. A 130/80 and the 120 sizes both require a 2.50-3.00 rim width. The 110 size does specify fitment onto a 2.15 rim, though. So, this size should work properly on the stock rear 750F rim.
My current favorites are Metzler tires. Alas, their web site has obscurred or eliminated the rim width specs. that I once found there. However, they do have the Lasertec 3.25 19 front which most certainly will fit your rim. And then there is the ME77 4.10 18 rear that I'm pretty confident will work on your rims properly, too.
Why is is rim width important? Sidewall support and tire tread crown shape. Pinching the tire bead together at the rim seat makes the tread arch more pronounced. If you run a lower tire pressure to recover the designed contact patch, then the side wall stiffness is poor as the rim support is inward, instead of under the sidewall. This gives you the squirmy butt feel a low tire would give. If you raise the tire pressure to stiffen the sidewall, this makes the tread crown more pronounced, and the tread wears rapidly in the center. Overall tire mileage diminishes, and handling issues develop as tire flattens the tread crown through wear. When the oversize tires are new, there are no feel problems, as the tread is still nicely arched. Later the issues creep in.
New tires always seem to feel better than the old tires. And, just because there is enough clearance within the fenders does not mean the tire fits properly, though this is an important consideration. Personally I want tires that will work well throughout their operational life. I've had oversize tires on my bike (120s on a 1.75 or 2 inch rim both Dunlops and Metzlers) that were brilliant when new but wore rapidly in the tread center and handled ever poorly as they wore down (lots of added counter steer leaned over and head shake when upright, both cured with a new tire).
I would prefer to take the manufacturers recommendation for tire fitment, over some people's advise. Sometimes, they (the manufacturer) actually knows more about the products they offer than buyers and users.
Cheers,
