Regardless of tires there will always be a situation where they cannot be spun on any tractor. Universally, to my knowledge, this will happen first with a fluid drive. This is because you dont have a mechanical connection between engine and wheels - the connection passes thru fluid and fluid pressure is limited by a relief valve. So the torque to the wheels is limited by the relief setting and the displacement of the hyd motor being driven and then the final drive [geared] ratio to the wheels. This is the ratio you select when you change ranges.
With an HST, regardless of the relief setting the engine cannot be stalled without abusive operation of the pedal because you are varying the displacement of the hyd pump. Without the relief, infinite forces would be available in any range just by adjusting the pedal, and thereby pump displacement, so the HP driving the pump would deliver whatever pressure required to give the torque needed at the wheels. Of course the pressures would have to be proportionately higher in the higher ranges to get the torque needed at the wheels. Well, darn it anyway, we cant just keep kicking the forces up thruout the powertrain like that. We have to limit each part according to its ability. We limit the pump/motor with a protective relief valve, and we make the gears in the ranges amply strong to take the [thereby limited] hyd motor torque. We adjust the range gear ratios to give usable speed and wheel torque combinations. This part of the hst tractor is exactly analogous to a gear tractor if you recognize that each gear ratio is a range. Not surprisingly, on modern gear tractors, that usually have at least 8gears, the ratios are much more closely spaced thruout a conventional tractor min/max range of travel speed, than they are with hst which has just 2 or 3 gears/ranges to cover the full range of conventional speeds. So you dont find a lo range on an HST as low as 1st on an 8 speed. The hst will go faster in 1 of 3 than the gear in 1 of 8. That means the HST is effectively in a higher gear. And that means that the HST cannot deliver as much torque to the wheels if the relief valve is set safely. To realize this you have to take a close look at the interaction of the engine, pump, relief valve, and motor. The relief has to be set at a level low enuf so that it will relieve if the tractor is stopped dead in a pull. If it didnt something requiring repair would give. The wheels would spin? In lo range maybe, but not in hi. If the relief doesnt give and the wheels dont spin the engine stops dead. This kind of impulse must be prevented. For maximum and "safe" power delivery it is done by setting the relief at a pressure that will stall the full engine power gracefully at full pedal. Somewhat more than full engine torque is being delivered to the pump in this situation. The hyd pressure seen here is as high as it can go in the protected transmission. This pressure defines the max torque available from the motor that is driving the final ratio. Changing the flow with the pedal has only a tiny effect on this torque as the engine bogs. Because of the relief, backing off the pedal to let the engine work easy does only that - the pressure and therefore torque limit at the motor remains the same.
larry
