I can check the real life size of HO Box Cars etc.,with my General Model Railroad Reference Rule,but lately buying several different brands of HO Vehicles,I find there are varying styles of HO. So far,the Classic Metal Works(plastic models) look to be right. The GMC Greyhound Coach,and the Chevrolet Pickup are comparable,where Herpa Ford 1950 Station wagons look a bit big ?.They are not too bad,butI would like advice on how to do a cross reference with my HO Rule.
Not sure what your question is. If you know the dimensions of the prototype vehicle, then the same HO rule you use for your boxcars can be used to check the model.
If you can find a source of correct dimensions for the vehicle, all you need to do is divide that number by 87.1. For modern vehicles, you might be able to measure the prototype. The real problem is in finding the correct dimensions for something more obscure or outdated.
Seek and you shall find:
Database of automobile and light truck dimensions 1971-2013
http://www.aras360.com/resources/vehicle-specs-database.html
or this site - Complete Catalog of Cars produced worldwide since 1945
http://www.automobile-catalog.com/
Classic cars 1910-1975
http://www.classiccardatabase.com/
and still another site - Worldwide 621 car marques (Do you need the dimensions of a 1913 Adams, a 1968 Zaz, or a 2008 Chery?)
http://carspector.com/
I tried to sell my two cents worth, but no one would give me a plug nickel for it.
I don't have a leg to stand on.
Thank you to all contributors.I have a starting point.