1. Back in the late 1950's, went deer hunting many times whereby we had to travel a mile & a half on a much traveled muddy & sloppy farm road to get to the woods.
2. Having (11) spare tires back then, put all 21" tires on my 1930 Coupe.
3. Coming back before dark at about 25 mph, on days when it rained all day, would always pass up about 10 or so unhappy hunters with their four (4) 4-wheel drive jeeps stuck in the mud with their 600-16 tires.
4. Their jeeps could not move forward or backwards while the jeep's "entire" undercarriage rested on & was dragging in the slop -- on a Model A, I had already experienced that only the slim front axle & about half of the rear axle is causing slight drag forces in the soft slop -- remainder of undercarriage was much higher in elevation.
5. As correctly mentioned in reply #4 above, the narrow 21" tires cut into the slush & hit firmer ground below.
6. Also when driving in mud, when tires begin to spin, first give it a little gas for about (2) seconds to spin faster, then up let off of the accelerator & the tires begin to grip on deceleration.
7. Repeat above step 6. every time one loses momentum & begins to spin.
8. Had a set of tire chains under the seat but did not have to use them often ...... that is on this particular road ....... but used them other times. Still have the chains.
9. Had to tell the land owner at the end of the road to get his tractor to retrieve the stuck jeeps in the dark.