Bill, Many people use 10 gauge (I have myself) and get by just fine. On sites such as here you often hear non professional electricians and non professional engineers proclaim 10 gauge is rated for 30 amps PERIOD, which is indeed correct for many many situations.
HOWEVER if you dig deep and study NEC ampacity charts, you see the ampacity depends NOT ONLY on wire gauge, BUT ALSO if the conductors are in free air,,,,,,, or buried in earth,,,,,,,,,,or jacketed,,,,,,,,,, or enclosed,,,,,,,,,,, or if in raceway,,,,,,,,,,, and if so how many,,,,,,,,,,, and the insulation and ambient temperature,,, and if copper or aluminum.
Another consideration is acceptable voltage drop, but in tractor applications, its NOT like you're running 50 or 100 feet of wire, its likely only 5 to 10 feet max so that becomes less critical. If you look up voltage drop for 55 amps at say 8 feet for 6 or 8 or 10 gauge wire you will see the difference. Sure bigger wire = less voltage drop, but for ONLY say 8 feet its likely NOT a game changer (But I didn't look it up so no freaking warranty and dont anyone have a calf now)
Another consideration is the duty cycle and if the battery charging circuit classifies under the NEC as a "continuous load". If not wire gauge recommendations vary.
Another consideration, the battery (depending on its condition and state of charge) likely isnt going to draw any 55 full amps
for long extended time periods. It may draw 55 a short time then back down to 40 then 30 then 10 amps subject to load.
THAT ALL BEING SAID AND AS A RETIRED ELECTRICAL DISTRIBUTION DESIGN ENGINEER I would consider 10 Gauge as the absolute minimum,,,,,,,,,,,, 8 Gauge as a happy medium,,,,,,,,,,,,6 Gauge may be overkill but wouldn't hurt a thing, the less voltage drop the better. I doubt if you will realize all that much difference if you used 6 or 8 or 10 gauge.
A final consideration, for no more then the cost and length of run it wouldn't hurt a thing to use 6 Gauge stranded insulated copper wire and I'm sure that will suffice given ALL those considerations I listed above (gauge, length of run, enclosures, insulation, free air or enclosed etc etc)
SUMMARY AGAIN, its my opinion 10 Gauge would be minimum and will work,,,,,,,,8 Gauge has some less voltage drop, 6 gauge has even less voltage drop. For no more then the cost even if overkill, bigger wire sure wont hurt anything.
We report you decide
John T Too long retired electrical engineer and rusty on all this so no warranty, use what you please and you get no argument from me.