3speed

My bike has 50mm wide rims and 2.125" Felt "Circuit" tires. The tires on these rims come in at 63mm at the widest point, which is almost 2.5 inches. I don't know if that's just standard tire sizing variance, or due to the rims. They seem heavy and slow to me, so I'm looking for something fat and fast, but I think most manufactures post sizes based on narrower rims. I have ~1.5cm clearance between these tires and fenders at all points. What size range do you think I should be looking at? Any chance I could fit a tire labeled at 2.5", or would that likely be too big? I'd like to go as big as possible.

Sixty Fiver

Seating wider tyres on the widest rim possible improves their performance... using too narrow a rim pinches the tyre inwards at the bead and at lower pressures the tyre can become squirmy while running the same tyre on a wider rim allows for better performance at lower psi.

There was a trend when mtb rims were almost as narrow as road rims but ran wider tyres, although a narrower rim works well on a skinny road tyre to lower drag, mounting a fatter tyre on these skinnier rims eliminated that advantage due to the greater frontal area.

Although a wider tyre will increase drag by a small amount, a well designed tyre with better tread compounds and a well made casing can still roll out rather quickly.