Lionel 6-12808 O27 Kenworth Tractor and trailer

I checked the wheelbase for the prototype tractor which is about 124 inches.

That would be a very, very short wheelbase conventional tractor, even with a single rear axle.

How are you measuring the wheel base?  Are your measuring from the center line of the front axle to the center line of the rear tandem axle?  Three and 13/16 inches equals 183 inches in 1:48 scale and would still be short for conventional tractor with tandem rear axles and a sleeper cab.  Is the Lionel KW 2 inches wide over the running boards or fenders?  If so you have a 1:48 scale model.

BTW, I believe that green tractor is a T-800. 

If you wanted something a little larger New Ray has a construction set this year in 1/43

Link

 You can get the truck and trailer individually as well.

Other link

Many of the Lionel tractor-trailers were 1:64 scale.  I think they looked noticeable too small to be on a layout.  The New Ray truck models Garfield showed above are very nicely priced and good models, but about 1:43.  Frankly they are a bit too big for me: I think 1:43 is a good scale for cars on a layout, but big trucks in that big scale look a bit too big for my tastes.  While it is possible to find exactly 1:48 scale big rigs but they are rare (I've found a nice Peterbilt but not a Kenworth), I've settled on 1:50 scale, of which there is a huge selection and you can find almost any brank truck from any era if you look. The easiest place to search is to go to the Toys and Games section of amazon and use their search for "1:50 truck" or "1:50 Kenworth" and get several thousand really nice offerings, or Buffalo Road diecast (constructionscalemodels.com) has a monstrous selection of 1:50 construction, trucking, and military vehicles.