I put the question simply to make you consider the validity of your original question.
I said nothing in my question about a single ratio transmission.
If the vehicle was a cricket pitch roller, then an engine of 3hp might suffice. A forklift might use 20hp, whilst a town car with a top speed barely above 30mph might get away with only 10hp. A lightweight sportscar would get interesting at around 60hp and a Le Mans prototype might be 500hp.
The point is that that it is not only the weight, but the top speed and acceleration that determine the power requirements for an ICE vehicle. The power requirements for an electric vehicle are different, both because of the different delivery curves and becuase of the way that motor/drive systems are rated, but they are effected by the same factors.
The electric race car I used to be involved with weighed about 1.5 Ton and used a motor a motor rated at something like 10hp (continuous). One of our rival teams with the same Porsce 914 base used a motor rated at 20hp (continuous). Their top speed was something like 90mph, whilst ours was 135mph. Their motor ran pretty hot (you could smell the varnish), whilst ours ran only warm (you could keep you hand on the casing indefinitely). They used 2 and sometimes 3 gears (a disadvantage as the shift on a 914 is truly awful) whilst we could get away with a single ratio.
If you only knew the stated ratings, which motor would you think would work best?
Their motor was an ADC FB1-4001 and was pretty much identical to our ADC FB1-4001. The only real difference between them was the spec. plate riveted to the side. Ours was rated at 72volt and theirs at 144volt.
They were running a 144volt battery pack, whilst we were at 288volt, although we limited the voltage at the motor terminals to not much more than 200. What our rivals really struggled to understand was how we were using about 50% lower peak power (motor input) to go considerably faster.
You expect me to come up with a rule of thumb that will determine the motor power (as displayed on the spec. plate) needed to propel a given weight vehicle at an unknown speed and with an unknown hillclimbing/acceleration ability.
I'm not guarding secret knowledge or deliberately trying to obstruct you, I simply don't know how to give you the answer you want, and I don't think anyone else does.
It reminds me so much of the attitude of older journeymen when I was an aprentice metalworker 50 years ago.DO NOT ASK DIFFICULT QUESTIONS!
Don't ask complicated questions unless you're prepared to accept complicated answers, or get questions back in return.