The Federal Highway Administration will look at eight types of tractor-trailer combinations in its comprehensive study of truck sizes and weights.

The agency will compare six variations to two familiar control configurations, the 5-axle, 80,000-pound workhorse, and the 6-axle, 80,000-pound twins combination of a tractor pulling two 28-foot or 28.5-foot trailers.

The first variation will be the 5-axle rig with a gross weight of 88,000 pounds.

The second will be the 6-axle, 97,000 unit that some in the industry are promoting as the logical expansion of the long-time federal size and weight limits. The third will be a 91,000-pound version of that unit.

The fourth will be a 6-axle, 80,000-pound combination of a tractor pulling two 33-foot trailers.

The fifth: a 7-axle, 105,500-pound combination of tractor and three 28-foot or 28.5-foot trailers.

And the last will be a 9- or 10-axle, 129,000-pound combination with three 28-foot or 28.5-foot trailers.

These details emerged from a webinar the agency conducted Wednesday. Look for more news from the webinar on Thursday.

The agency is halfway through a two-year study that is intended to give Congress the information it needs to decide on possible changes in current standards. The study is due to be finished by next November.

The agency originally intended to take comments on the study until Jan. 3, but now has extended that deadline to Jan. 17.