Is the time the rear wheel free spin an indication of any drag? I'm just wondering cause on my road bike the rear wheel seems to spin forever while on my surly ogre do it all bike it's not a match... not even a third of the time I think.... they are very diffent though, road bike has slick tyres 25c 32 spokes and a hub that's done several thousand of miles, on the other hand, the ogre has marathon touring 50mm tyre, 36 spokes but same rims.... hub is relatively new with about 500 Mls..... also, it has a fender (with close to 1" gap between tyre and fender) and a disc brake.
Yes, it is an indication of drag, but not significant drag. To emphasise the point, the drag "measured" by that method is totally insignificant.
Wheel drag comes from two sources; bearing drag and aerodynamic drag.
As for bearing drag, that's again broken up into two components; lubricant viscous drag and bearing element rolling drag. Both these vary. Grease drag reduces between services, since grease is a consumable and ball drag varies with preload. Since there is only one optimum preload, you don't want to fiddle with that and since you don't want to replace grease every day, you fill the bearings appropriately.
Aerodynamic drag is mostly from the spokes but also from rim and tyre rotation.
Wheel drag is infinitely small compared to "package drag". By that I mean the drag caused by the entire package of you and a messy buff* body (your bike) moving through the air at 20kph.
It is very easy to find out if your hubs have too much drag. Ride a few hundred meters and stop and feel if it is hot. If it blisters your fingers, there is drag. If it doesn't, drink a beer and watch the butterflies.
* Buff in this case means non-aero, it doesn't refer to your guns. Guns is slang for biceps. Biceps are those two....nevermind
