cause of drag?
 

i have a 66 mustang coupe and was wondering what would be causing alot of drag. my car seems to slow quite fast if im not always on the gas. i dont think its the wheel bearing because those seem to be good. what about from the driveshaft. iv looked at it and it looks really dirty with just road gunk and dirt. any special stuff i can try on that. also what about transmission. if i go downhill in drive with foot off gas i slow pretty fast. if i do the same but in neutral i seem to coast much better and not slow as fast. anything i should try for this? anything i can try to help remove any friction? thanks in advance

I don't think you'll find the driveshaft causing drag.

More then likely I would look at the brakes for drag.

The transmission should have some drag in Drive. The newer O/D transmission have very little drag, unlike the older unoverdriven units, C-4, C-6, FMX.

Put the car in air and try hand spinning the wheels. That will show you where the drag is.

You said that when in gear you slow down faster than when in neutral. That could be due to the axle ratio. The lower the gear (numerically higher) it will slow you down faster than a higher gear. When in neutral the axles are free-wheeling and the ratio doesn't affect how fast you slow down.