PROFILE of Nico71

Thanks for your feedback ! The instructions has been made with LDcad+Lpub3D, both used Ldraw parts library.

Hello, Using the wheel is the good way to start the scaling indeed, then working on the other dimensions with correct proportions. Of course you can stretch a bit to fit the LEGO elements or give a different look to you build. But this is accurate. 
A 1:15 scale car is about 30cm long in LEGO, for 4.5m in real (15*30cm=450cm), which is the case for the Corvette ZR1. I think you make your calculus wrongly.
You can also calculate with the width of an object in LEGO is it is wide enough. For instance the truck is 15stud wide (without the mirror) so 12cm. A truck is 2.5m wide, so 250/12=20 so scale is 1/20. Which is correct also with the tire of the wheel which is 49mm for about 1000mm in real (1000/49=20.40 -> 1/20).

Happy that you like ! Nice modification for the rally car !

It depends of the size, proportions of the original model and primarily the size of  wheels, but yes most of transporter car are 1:15 (1/15 for about 650mm wheel) while the truck is more about 1:20 (49mm for 1000mm wheel). On your model, the problem is that the wheel are too big, or the body too small. You can try 37mm wheel to look more 1/20 but not sure the other proportion will match. It is not a big deal, just a big cartoony and it works on the car transporter, but scaling is important to look realistic (not too much scaling has you have to adapt also to the lego, with visual weight design).

Nice addition to the new supra from LEGO, but this is not 1/20, more 1/15 (650mm wheel in real, 43mm in LEGO).