HP values on jet engines may or may not be nonsense. The HP actually changes with speed as jets are (sort of) constant thrust. Power = Velocity*Force, so that HP rating is only good at a certain speed.
Indeed, because power is the effect of the force.
To compare the power of a jet engine to the power of a piston engine you need to pick a speed where you want to make the comparison.
And what may be good to know as well is that it's not power that makes the car accelerate, it's a force. For a piston engine car the force accelerating the car is the torque multiplied by the gear ratio and divided by the radius of the wheel. Power, on the other hand, tells you the effect of the torque. Torque at high engine speeds is more valuable than torque at low engine speeds, because with high engine speeds you can run in a shorter gear, which multiplies the torque more than a taller gear and produces a greater force at the wheel. The value of torque is what is expressed by the power figure.
For a jet engine car the force accelerating the car is the thrust.
You
can make a comparison of the power of a jet engine and a piston engine, but the piston engine is capped at a certain engine speed and won't go any faster, while the top speed of a jet engined vehicle depends on aerodynamic drag, so it's kind of unfair.
So a comparison would probably look something like this, with the piston engine car running in a single gear:
Another method could be to compare against the potential of the piston engine at any given gear ratio:
So in this example, with a short gear ratio even a relatively low powered car can produce greater force and power at low speed than the jet powered car (at 10 m/s) but once the piston car reach a bit of speed and need to shift into a taller gear, the jet powered car flies past, and its power figure keeps climbing until the car reaches its top speed.
The piston car, assuming infinate gears, maintains constant power throughout the acceleration thanks to its gearbox, but as the force is multiplied by the gear ratio, and the gear ratio gets smaller and smaller, the force drops a lot.
Edit: So, what would be better at the farm? A jet engine tractor or a piston engine tractor?
The piston engine would be far superior, due to the ability to change its gearing. The jet engine would have to use brute force to drag the plow through the field, while the piston engine would use the leverage provided by the gearbox.