The magic formula for tyres
Well, there isn't one. But you know that.
I ride on 26" wheels, on an oddball bike that can take up to 2" tyres.
My 'fast' riding has been on 1.3" sport contacts (fast slicks at 85psi).
My current riding is on 1.75" Randonneur Cross (coarse treaded at 70psi).
There is about 1mph difference between them, for the same effort.
I really notice differences between them in three circumstances.
Coasting on the flat. The Sport contacts roll so easily it's almost like I'm still pedalling. The Randonneur Cross's start slowing immediately.
Rough tarmac and stones. If I hit a stone on the sport contacts, the bike skips and slides off the stone. With the Randonneurs, it just bumps a bit. On rough tarmac, the Sport contacts vibrate. the randonneurs rumble a bit.
Cornering at speed. The treaded tyres joggle sideways unpredictably. Quite disconcerting.
It's surprising how much more tiring it is to push along the fat treaded tyres. Not noticeable on one short ride, but over a hundred miles or so, or many successive 25mile rides, the treaded tyres do wear me down.
OTOH, the fatter tyres are far more comfortable, and I don't worry about the bike sliding on stony roads.
I'm going to try a compromise: 1.5" Panaracer Pasalas, with a slight amount of tread. Maybe *they* will hold the magic formula.