Acherly, it snot tork it's thrust which is tractive force at the
wheel/pavement interface.  Geeky, yep, but its always just fizzix.  So
figure your torque at the output shaft of the motor.  Multiply by the
overall equivalent final drive ratio in each gear ( gotta know the gear
ratios)  Measure the rollout of the wheel and tire to get the tire rolling
radius.  Convert the equivalent torque in lbs-ft to thrust in lbs by
multiplying by the radius in feet of your tire.   Yeah, it's a bit more than
one probably  but lets say you have 100lbsft available and you have a final
drive of 5.0:1  that will give you 500lbs ft of torque at the axle if you
have 24 inch rolling diameter wheels you have a rolling radius of 1 ft
yielding 500 lbs of thrust.  This means you can handle anything that has
~400lbs of drag.  With 100lbs left over degraded by say 15% losses to yield
an 85lb thrust margin.  NB the drag is not the same as the mass of the
thingy on the hitch.

You get the idea, plug in your measured data/numbers and you can figure out
what you might be able to pull, not tow.  If you put a spring scale
calibrated in pounds onto the hitch and try to pull a building the motor
will stall or clutch slip.  The reading on the scale is the thrust.  Better
to use your Keuffel & Esser Log Log DeciTrig instead.  You do have one, or
maybe a Post Versalog or your favorite multiplier that has pi all over it.

P