This criterea is an important part of efficiency ; directly affects
vehicles' mpg, should be disclosed as required by law.
i] brake bind : e.g. my 1985 Nissan Sunny 130Y assembled in msia
  had springs fitted in a wrong way ( the springs force idle brake pads
  onto brake discs ), so the front wheels could barely spin before I
  removed these stupid springs.
ii] size / height of tyres : the taller is a tyre, the lower will be the min
    tow force needed to overcome the same brake bind, so cheapo cars
    with short tyres will need a higher min tow force.
iii] toe-in : e.g. my 1990 honda accord's prescribed rear wheels' toe-in
    kept rolling resistance very high, before I ignored these toe-in.
iv] low rolling resistance tyres : e.g. Michelin's "green" tyres have lower
    rr than Goodyear / Dunlop 's.  So if a vehicle has cheap tyres with
    high rr, then its specification should reveal a higher min tow force
    required.
v] lubricant in wheel bearing : now modern lubricants ( e.g. synthetic
   oil / fullerene / semi-paraffin ) can be 2x as slippery as grease, so if
   a manufacturer still puts cheapo grease into bearings, this would
   cause a higher min tow force.
vi] nitrogen in tyres : will significantly reduce rr esp for big / high
    profile tyres ( like lorries' ).  So manufacturers should not be allowed
    to ignore this fact, @ the expense of mpg & consumers' CO2
   discharge.