HST are different than gear tractors.
gear / manual transmissions = if you put to much of a load via high gearing you stall and kill engine. pretty simple.
on HST / hydrualic transmissions = there is an interneral pressure relief valve. so instead of stalling and killing engine. the pressure relief valve opens up. removing power.
with HST, you need to drop it to low range and work your way up till you find and have experience of both power / torque and speed / MPH of unit.
with HST, low range, peddle down just a little bit = most power/torque and slowest speed / MPH
with HST, low range, peddle down all the way = less power/torque and more speed / MPH
with HST, highest range, peddle down all the way = least amount of power/torque and highest speed / MPH
other words...
if you have say mid range on gear shiftier and you have peddle down all the way and barely moving along. let up on the peddle. and see if you begin going faster. if you still are not going faster. drop down to next range. and try the peddle and see what gets you max speed / MPH
tractors spinning wheels = not a hot rod spinning tires on pavement and dirt. you do not want to spin your tires. you want as much possible traction as you can get.
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above is general blah blah. but from own personal experience should apply to HST transmissions.
it would be nice if there was a "DUM DUM light" that came on letting operator know the pressure relief valve opened up on the HST transmission giving operators some sort of notice that they might need to drop down to a lower range / ease up on the peddle some to get better power/speed overall.