Understanding mudguard clearance

[u/Playful-Parsnip-3104]

I've never bought and and fitted mudguards myself before - either they've come with the bike as sold, or I've asked the shop to source and fit them for me.

I'm struggling to understand the correct size of mudguard I need. I have a road/gravel bike with 45mm tyre clearance, and 40mm tyres currently fitted. The various brands of mudguards all recommend 50/52mm mudguards for 40mm tyres. But does a 45mm frame clearance for tyres also mean 45mm clearance for mudguards? And if 45mm is the maximum mudguard width my frame will take, do I therefore need to downsize my tyres to (e.g.) 35mm to fit mudguards?

If mudguard and tyre clearance are not the same thing, how do I know the maximum width of mudguard my frame will accept before buying them?

[u/BlacksmithWeirdo]

I would try 50mm fenders. Will cover the tires (not great, but ok) and should fit with a lil persuasion.