Rocker

The Tag axle also offers you an extra set of brakes so you have 6 rather than 4 and those brakes will also cut your braking distance by up to 50 feet.

Your fuel mileage will vary just like anything else. The size of the engine, The transmission. Cost again depends on the options Tolls. Some coaches have a tag axle lift so in reality if the tag is not touching the road surface you pay for two. Heavy duty dump trucks do it all the time. With a tag you can increase your GVW, Most tag axles unlock at slow speeds to give you 4 wheel steering while parking at a curb. I lifting tag also makes turning much easier since it changes the pivot point of the turn