When I bought my CB500 secondhand in 1980, it had a 3.50 tyre front (Conti RB2) and a 4.00 (K112) rear, both on standard rims. Although the front tyre did not rub, the next time I needed a new one I went to 3.25 as I fail to see what the advantage of a wider tyre is. Since 3.75 was not available, I've always stayed with 4.00 rear tyres. No problem at all. The standard rear rim can have a 3.50 or a 3.75 or a 4.00 tyre no problem.